Water absorbing agent, water absorbent core using the agent, and manufacturing method for water absorbing agent

ABSTRACT

A water absorbing agent of the present invention has an internal crosslinking structure obtained by polymerization of a water-soluble unsaturated monomer. The agent satisfies conditions (a) to (d): (a) the agent contains water-insoluble inorganic particles at an amount of from 10 ppm to 1,900 ppm inclusive; (b) the agent contains 5 mass % or less particles which have such a size that they can pass through a sieve having a mesh opening size of 150 μm; (c) the agent has an absorbency against a pressure of 4.83 kPa (AAP) of 18 g/g or more; and (d) the water-insoluble inorganic particles reside on a surface of the water absorbing resin or near the surface.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Division of application Ser. No. 12/294,328 filedon Sep. 24, 2008, the entire contents of which are incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to water absorbing agents, water absorbentcores using the agents, and manufacturing methods for the waterabsorbing agents. Specifically, the present invention relates to waterabsorbing agents, water absorbent cores, and manufacturing methods forthe water absorbing agents which are suitably applicable to disposablediapers, sanitary napkins, so-called incontinent pads, and othersanitary/hygienic materials.

BACKGROUND ART

Water absorbent cores containing hydrophilic fiber, such as pulp, and awater absorbing resin particles are widely used conventionally so thatsanitary/hygienic materials, such as disposable diapers, sanitarynapkins, and incontinent pads, can absorb body fluids. The waterabsorbent core is used in sanitary/hygienic materials, such asdisposable diapers, sanitary napkins, and incontinent pads, to absorbbody fluids.

There are recent demands for these sanitary/hygienic materials to bereduced in thickness for better usability. Therefore water absorbentcores are manufactured with a decreasing ratio of hydrophilic fiber,which has a relatively low bulk density, and an increasing ratio ofwater absorbing resin particles, which exhibit excellent waterabsorption and a relatively high bulk density. The relative quantity ofwater absorbing resin particles used in the water absorbent core ishence increased, which in turn reduces the thickness of thesanitary/hygienic materials without compromising water absorbency andother physical properties.

The ratio of the hydrophilic fiber may be decreased, but not furtherbelow a minimum quantity required. For further reduction in thickness ofthe sanitary/hygienic materials, the physical properties of the waterabsorbing resin particles need to be improved. Examples of such physicalproperties of the water absorbing resin particles include centrifugeretention capacity, saline flow conductivity, absorbency againstpressure, fixed height absorbency, mass median particle size, and liquiddistribution velocity. These physical properties of the water absorbingresin particles need to be in predetermined ranges or excellent inactual use.

In, for example, a diaper which contains a sanitary/hygienic materialwith a high proportion of water absorbing resin particles, the waterabsorbing resin particles absorb water and changes into gel or a similarcondition. That may lead to a phenomenon called gel blocking. Thephenomenon reduces liquid diffusibility of the sanitary/hygienicmaterial.

A method of adding inorganic particles to the water absorbing resinparticles has been proposed to improve the physical properties of thewater absorbing resin particles and address the gel blocking problem.According to the method, the inorganic particles are present between thewater absorbing resin particles, thereby preventing the water absorbingresin particles from aggregating. The gel blocking problem is mitigated.

Patent document 1 discloses a water absorbing agent composition as waterabsorbing resin particles used with inorganic particles. The compositioncontains crosslinked, water-swelling resin powder and hydrophobicsuperfine particulate silica. The composition is intended to achievegood fluidity in powder form, not to get sticky when having absorbedmoisture (thus allowing for easy handling), and to show excellent waterabsorption and water retention capabilities.

Patent document 2 discloses a water absorbing polymer agent composition.The composition is prepared by sticking inorganic fine powder in asecondary aggregate state to the surface of a water absorbing polymeragent in coarse particle form. This particular method of preparation isa feature of the composition. The composition is intended to achieveexcellent fluidity and a high liquid absorption rate.

Patent document 3 discloses modified water absorbing resin particles foruse in sanitary products. The particles show an increased absorptionrate and causes less gel blocking. The particles are a crosslinkedpolymer of an unsaturated ethylenic monomer that has an acrylic acidand/or an acrylic acid salt as major structural units. The particles aretreated with liquid organic polysiloxane at normal temperature.

Patent document 4 discloses modified water absorbing resin particles.The particles show an increased absorption rate, and cause mitigatedmoisture-driven blocking and restrained dust production. The particlesare prepared by treating water absorbing resin particles with asilicone-based surfactant.

Patent document 5 discloses a water absorbing agent prepared frominorganic powder and water absorbing resin. The agent causes mitigatedblocking when having absorbed moisture, offers easy handling, and showsexcellent absorption properties under load. The inorganic powderexhibits a pH from 7 to 10, inclusive, when dispersed in a liquid, and aspecific surface area of 50 m²/g or more as measured by BET.

Patent document 6 discloses a water absorbent core containing a waterabsorbing resin and a hydrophilic fiber. The core exhibits an absorbencyunder load of 10 g/g or less for artificial urine 30 seconds afterabsorption is started and a water absorbency under load of 20 g/g ormore for artificial urine 30 minutes after absorption of water isstarted. The core is intended to achieve reduced thickness withoutcausing problems in actual use.

Patent document 7 discloses a water absorbent core containing a waterabsorbing resin and a hydrophilic resin. The water absorbing resinexhibits an absorption swelling pressure of 10,000 Pa or less forphysiological saline as the test solution and an absorption swellingpressure of 80,000 Pa or higher 300 seconds after absorption of water isstarted. The core is intended to achieve excellent liquid diffusibilityand a low level of liquid seeping.

Patent document 8 discloses a technique of using 3D spacers in thepreparation of a water absorbing resin.

Patent document 9 discloses a particulate water absorbing agentcontaining water absorbing resin particles and a liquid permeabilityimprover. The particles are prepared by polymerizing (crosslinking) amonomer of an acrylic acid and/or its salt and further crosslinking thesurface of the resultant irregularly pulverized particles. The agent isintended to be superior in both physical properties: capillary suctionforce and liquid permeability.

Patent document 10 discloses a water absorbing resin composition thatcontains a monomer with carboxyl groups and fumed silica for improveddeodorizing effects and fluidity.

Patent document 11 discloses a technique of adding a liquid permeabilityimprover to a water absorbing resin in the preparation of a waterabsorbing resin.

-   [Patent Document 1] Japanese Unexamined Patent Publication    56-133028/1981 (Tokukaisho 56-133028; published Oct. 17, 1981)-   [Patent Document 2] Japanese Unexamined Patent Publication    64-4653/1989 (Tokukaisho 64-4653; published Jan. 9, 1989)-   [Patent Document 3] Japanese Patent 3169133, Specification    (registered Mar. 16, 2001)-   [Patent Document 4] Japanese Unexamined Patent Publication    9-136966/1997 (Tokukaihei 9-136966; published May 27, 1997)-   [Patent Document 5] Japanese Unexamined Patent Publication (Tokukai)    2000-93792 (published Apr. 4, 2000)-   [Patent Document 6] Japanese Unexamined Patent Publication (Tokukai)    2003-88551 (published Mar. 25, 2003)-   [Patent Document 7] Japanese Unexamined Patent Publication (Tokukai)    2003-88553 (published Mar. 25, 2003)-   [Patent Document 8] U.S. Published Patent Application 2002/0128618,    Specification (Sep. 12, 2002)-   [Patent Document 9] Japanese Unexamined Patent Publication (Tokukai)    2004-261797 (published Sep. 24, 2004)-   [Patent Document 10] Published Japanese Translation of PCT    Application (Tokuhyo) 2003-500490 (published Jan. 7, 2003)-   [Patent Document 11] International Application Published under PCT    WO2004/69915 (Aug. 19, 2004)

DISCLOSURE OF INVENTION

The conventional techniques listed above have a problem that they cannotdeliver a water absorbing agent with necessary physical properties aswater absorbing resin or achieve limited dust production in themanufacture of water absorbing resin.

Specifically, water absorbing resin is required to exhibit good physicalproperties (centrifuge retention capacity, saline flow conductivity,absorbency against pressure, fixed height absorbency, mass medianparticle size, liquid diffusibility, etc.) in the actual use of thewater absorbing resin. Conventional technology has so far failed toachieve sufficient values with these physical properties. One factor inthe failure is the trade-off between centrifuge retention capacity andsaline flow conductivity, both of which are important physicalproperties for water absorbing resin: if either of the physicalproperties improves, the other suffers. It is difficult to achieve goodvalues with both of the physical properties.

In addition, in conventional technology, if inorganic particles areadded to the water absorbing resin, a new problem arises that dust couldbe created from the inorganic particles. The dust may reduce themanufacturing efficiency for the water absorbing resin, degrade thephysical properties of the water absorbing resin, or raisesafety/hygienic concerns. Especially, when the inorganic particles isused in 0.2 mass % or more to the water absorbing resin, dust is likelyto occur due to the relative abundance of the inorganic particles used.

The present invention, conceived in view of these conventional issues,has an object of providing a water absorbing agent and a water absorbentcore which exhibit excellent physical properties and are unlikely tocreate dust, and also providing a method of manufacturing the waterabsorbing agent.

A water absorbing agent of the present invention is, in order to solvethe problems, characterized in that it contains water absorbing resinparticles with an internal crosslinking structure obtained bypolymerization of a water-soluble unsaturated monomer. The agent isfurther characterized in that it satisfies conditions (a) to (d) below:

(a) the agent contains water-insoluble inorganic particles at an amountof from 10 ppm to 1,900 ppm inclusive;

(b) the agent contains 5 mass % or less particles which have such a sizethat they can pass through a sieve having a mesh opening size of 150 μm;

(c) the agent has an absorbency against a pressure of 4.83 kPa (AAP) of18 g/g or more; and

(d) the water-insoluble inorganic particles reside on a surface of thewater absorbing resin or near the surface.

The water absorbing agent of the present invention is preferably suchthat the water-insoluble inorganic particles account for 10 ppm to 990ppm, inclusive, of the agent.

The water absorbing agent of the present invention is preferably suchthat the water-insoluble inorganic particles contain amino groupsresiding at least on the surface of the particles.

The water absorbing agent of the present invention is preferably suchthat: the water-insoluble inorganic particles are silicon dioxide; andthe silicon dioxide has, on a surface thereof, residual silanol groupsat a concentration of 1.7 SiOH/nm² or lower.

The water absorbing agent of the present invention is preferably suchthat the water absorbing agent has a saline flow conductivity (SFC) of30 (10⁻⁷·cm³·s·g⁻¹) or more.

The water absorbing agent of the present invention preferably has anabsorbency against a pressure of 4.83 kPa (AAP) of 20 g/g to 30 g/ginclusive.

The water absorbing agent of the present invention preferably furthercontains an at least trivalent water-soluble polyvalent metal salt at anamount of from 0.1 mass % to 1 mass % inclusive.

The water absorbing agent of the present invention is preferably suchthat the water-soluble polyvalent metal salt is aluminum sulfate.

The water absorbing agent of the present invention is preferably suchthat the water absorbing resin particles contain particles with a porousstructure.

The water absorbing agent of the present invention is preferably suchthat: the agent has a mass median particle size of 200 μm to 500 μminclusive and a logarithmic standard deviation, σζ, of a particle sizedistribution of 0.20 to 0.40 inclusive.

The water absorbing agent of the present invention preferably has aliquid distribution velocity (LDV) of 0.2 (mm/sec) to 10.0 (mm/sec)inclusive.

The water absorbing agent of the present invention preferably has anegative frictional electric charge.

The water absorbing agent of the present invention preferably contains300 ppm or less dust by mass.

The water absorbing agent of the present invention preferably containsdust in such an amount that the dust contains SiO₂ which is 50 mass % orless.

The water absorbing agent of the present invention is preferablyobtained by a method of manufacturing which involves the step of mixingthe silicon dioxide with the water absorbing resin particles aftergiving mechanical damage to the water absorbing resin particles.

The water absorbing agent of the present invention is preferablyobtained by a method of manufacturing which involves the step ofpneumatically transporting the silicon dioxide and the water absorbingresin particles after mixing the silicon dioxide with the waterabsorbing resin particles.

Another water absorbing agent of the present invention is, in order tosolve the problems, characterized in that it contains water absorbingresin particles obtained by polymerization of a water-solubleunsaturated monomer. The agent is further characterized in that itsatisfies conditions (A) to (D) below:

(A) the particles are, near a surface thereof, either crosslinked orcoated with a surface crosslinking agent which has at least one hydroxylgroup;

(B) the particles contain a polyvalent metal salt and water-insolubleinorganic particles at least either on or near the surface;

(C) the water absorbing agent has a mass median particle size of 200 μmto 500 μm inclusive; and

(D) the water absorbing agent contains 5 mass % or less particles whichhave such a size that they can pass through a sieve having a meshopening size of 150 μm.

The water absorbing agent of the present invention is preferably suchthat the polyvalent metal salt accounts for 0.01 mass % to 1 mass %,inclusive, of the water absorbing agent.

The water absorbing agent of the present invention is preferably suchthat the water-insoluble inorganic particles account for 0.001 mass % to0.4 mass %, inclusive, of the water absorbing agent.

The water absorbing agent of the present invention is preferably suchthat the water-insoluble inorganic particles are silicon dioxide.

The water absorbing agent of the present invention is preferably suchthat the water absorbing agent has a centrifuge retention capacity of 30g/g inclusive to 50 g/g exclusive, and a saline flow conductivity (SFC)of 10 (10⁻⁷·cm³·s·g⁻¹) or more.

The water absorbing agent of the present invention preferably containsdust in such an amount that the dust contains SiO₂ which is 50 mass % orless.

A water absorbent core of the present invention is, in order to solvethe problems, characterized in that it contains any of theaforementioned water absorbing agents.

A method of manufacturing a water absorbing agent of the presentinvention is, in order to solve the problems, characterized in that itis a method of manufacturing a water absorbing agent containing waterabsorbing resin particles obtained by polymerization of a water-solubleunsaturated monomer, the water absorbing resin particles having a massmedian particle size of 200 μm to 500 μm inclusive. The method isfurther characterized in that it involves the sequential steps of: (1)either crosslinking or coating the water absorbing resin particles neara surface thereof with a surface crosslinking agent which has at leastone hydroxyl group; and (2) mixing a polyvalent metal salt andwater-insoluble inorganic particles with the water absorbing resinparticles.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a cross-sectional view illustrating an AAP measurementapparatus in relation to the present example.

FIG. 2 is a schematic illustration of a SFC measurement apparatus inrelation to the present example.

BEST MODE FOR CARRYING OUT INVENTION

The following will describe the present invention in detail. The scopeof the present invention is however not limited by the description.Apart from the examples given below, the invention may be modified inother ways for implementation without departing from the spirit of theinvention. Note that in the present invention, “weight” and “mass” aresynonyms of “wt %” and “mass %” respectively. Throughout thespecification and claims, only “mass” and “mass %” are used. Thenumerical expression “A to B” refers to a range of more than or equal toA and less than or equal to B.

Abbreviations which will be used in the following description aredefined first. CRC is an acronym of “centrifuge retention capacity.” SFCis an acronym of “saline flow conductivity.” AAP refers to absorbencyagainst a pressure of 4.83 kPa. FHA is an acronym of “fixed heightabsorbency.” LDV is an acronym of “liquid distribution velocity.” D50refers to a mass median particle size. σζ is the logarithmic standarddeviation of a particle size distribution. Saline is an aqueous solutionof sodium chloride. 1 ppm is equal to 0.0001 mass %.

An embodiment of the present invention is now described. The waterabsorbing agent of the present embodiment contains water absorbing resinparticles. The water absorbing resin particles contain water-insolubleinorganic particles (hereinafter, may be referred to as “water-insolubleinorganic fine particles”).

Water Absorbing Resin Particles

The water absorbing resin particles used in the present embodiment areparticles of a water-insoluble, water-swelling, hydrogel-forming polymerprepared by polymerization of a water-soluble unsaturated monomer(hereinafter, may also be referred to as a “water absorbing resin”).

Concrete examples of the water-insoluble, water-swelling,hydrogel-forming polymer include partially neutralized, crosslinkedpolyacrylic acid polymers (Specification of U.S. Pat. No. 4,625,001,Specification of U.S. Pat. No. 4,654,039, Specification of U.S. Pat. No.5,250,640, Specification of U.S. Pat. No. 5,275,773, Specification ofEuropean Patent 456136, etc.); a partially neutralized, crosslinkedstarch-acrylic acid graft polymer (Specification of U.S. Pat. No.4,076,663); an isobutylene-maleic acid copolymer (Specification of U.S.Pat. No. 4,389,513); a saponification product of a vinyl acetate-acrylicacid copolymer (Specification of U.S. Pat. No. 4,124,748); a hydrolysateof an acrylamide (co)polymer (Specification of U.S. Pat. No. 3,959,569);and a hydrolysate of an acrylonitrile polymer (Specification of U.S.Pat. No. 3,935,099).

The water absorbing resin particles of the present embodiment arepreferably particles of a water absorbing resin containing a polyacrylicacid/polyacrylate-based crosslinked polymer obtained by polymerizationof a monomer containing an acrylic acid and/or salt thereof. In thepresent embodiment, the polyacrylic acid/polyacrylate-based crosslinkedpolymer refers to the crosslinked polymer obtained by polymerization ofa monomer containing an acrylic acid and/or salt thereof in at least 50mol %, preferably at least 70 mol %, more preferably at least 90 mol %.

Acid groups in the crosslinked polymer are neutralized in a ratiopreferably from 50 mol % to 90 mol % inclusive, more preferably from 60mol % to 80 mol % inclusive. The polyacrylate may be, for example, analkali metal salt, such as sodium, potassium, or lithium; an ammoniumsalt; or an amine salt. A preferred example is a sodium salt. The saltmay be formed in a neutralization before the polymerization, that is,through the neutralization of the monomer, or during or after thepolymerization, that is, through the neutralization of the polymer.Alternatively, any of the methods may be used together.

The polyacrylic acid/polyacrylate-based crosslinked polymer that issuited for use as the water absorbing resin particles of the presentembodiment may be prepared by copolymerizing another monomer, ifnecessary, in addition to the primary component monomer (acrylic acidand/or salt thereof). Concrete examples of the other monomer includeunsaturated anionic monomers, such as methacrylic acid, maleic acid,vinyl sulfonic acid, styrene sulfonic acid,2-(meth)acrylamide-2-methylpropanesulfonic acid,2-(meth)acryloylethanesulfonic acid, and 2-(meth)acryloylpropanesulfonicacid, and salts thereof; non-ionic hydrophilic group-containingunsaturated monomers, such as acrylamide, methacrylicamide,N-ethyl(meth)acrylamide, N-n-propyl(meth)acrylamide,N-isopropyl(meth)acrylamide, N,N-dimethyl(meth)acrylamide,2-hydroxyethyl(meth)acrylate, 2-hydroxypropyl(meth)acrylate,methoxypolyethylene glycol(meth)acrylate, polyethylene glycolmono(meth)acrylate, vinylpyridine, N-vinylpyrrolidone, N-acryloylpiperidine, N-acryloyl pyrrolidine, and N-vinylacetoamide; andunsaturated cationic monomers, such asN,N-dimethylaminoethyl(meth)acrylate,N,N-diethylaminoethyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylate,N,N-dimethylaminopropyl(meth)acrylamide, and quaternary salts thereof.The monomers, other than the acrylic acid and/or salt thereof, may beused in an amount of preferably 0 mol % to 30 mol % inclusive, morepreferably 0 mol % to 10 mol % inclusive, to the total amount of themonomers.

The water absorbing resin particles used in the present embodiment are acrosslinked polymer with an internal crosslinking structure. Theinternal crosslinking structure may be introduced to the water absorbingresin particles, for example, through self-crosslinking using nocrosslinking agent or by copolymerizing or reacting an internalcrosslinking agent containing two or more unsaturated polymerizinggroups and/or two or more reactive groups per molecule of the internalcrosslinking agent (the copolymerization or reaction of an internalcrosslinking agent is preferred).

Concrete examples of the internal crosslinking agent include polyhydricalcohols, such as N,N′-methylene bis(meth)acrylamide, (poly)ethyleneglycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate,trimethylolpropane tri(meth)acrylate, trimethylolpropanedi(meth)acrylate, glycerine tri(meth)acrylate, glycerine acrylatemethacrylate, ethylene oxide denatured trimethylolpropanetri(meth)acrylate, pentaerythritol tetra(meth)acrylate,dipentaerythritol hexa(meth)acrylate, triallyl cyanurate, triallylisocyanurate, triallyl phosphate, triallylamine,poly(meth)allyloxyalkanes, (poly)ethylene glycol diglycidyl ether,glycerol diglycidyl ether, ethylene glycol, polyethylene glycol,1,4-butanediol, propylene glycol, glycerine, and pentaerythritol;ethylenediamine; polyethyleneimine; and glycidyl(meth)acrylate.

Any one of the internal crosslinking agents may be used alone;alternatively two or more of them may be used. In view of the waterabsorption property of the obtained water absorbing resin particles andother factors, a preferred internal crosslinking agent must be acompound with two or more unsaturated polymerizing groups. The internalcrosslinking agent used accounts preferably for 0.005 mol % to 3 mol %inclusive, more preferably for 0.01 mol % to 1.5 mol % inclusive, mostpreferably for 0.05 mol % to 0.2 mol % inclusive, of the entire monomer.

In the polymerization, a hydrophilic polymer, such as starch-cellulose,a derivative of starch-cellulose, polyvinyl alcohol, polyacrylic acid(salt), or a crosslinked polymer of polyacrylic acid (salt), or a chaintransfer agent such as a hypophosphorous acid (hypophosphite) may beadded.

The monomer containing the above-mentioned acrylic acid and/or saltthereof as the primary component(s) can be polymerized by bulkpolymerization, reverse suspension polymerization, or precipitationpolymerization. Nevertheless, solution polymerization, using the monomerdissolved in water, is preferred in view of performance and ease incontrolling the polymerization. These polymerizations are described in,for example, the Specification of U.S. Pat. No. 4,625,001, theSpecification of U.S. Pat. No. 4,769,427, the Specification of U.S. Pat.No. 4,873,299, the Specification of U.S. Pat. No. 4,093,776, theSpecification of U.S. Pat. No. 4,367,323, the Specification of U.S. Pat.No. 4,446,261, the Specification of U.S. Pat. No. 4,683,274, theSpecification of U.S. Pat. No. 4,690,996, the Specification of U.S. Pat.No. 4,721,647, the Specification of U.S. Pat. No. 4,738,867, theSpecification of U.S. Pat. No. 4,748,076, and the Specification of U.S.Published Patent Application 2002/40095.

In the polymerization, a radical polymerization initiator, such aspotassium persulfate, ammonium persulfate, sodium persulfate,t-butylhydroperoxide, hydrogen peroxide, or 2,2′-azobis(2-amidinopropane)dihydrochloride, or an activation energy beam, such as anultraviolet or electron beam, may be used. In the case of using theradical polymerization initiator, redox polymerization may be carriedout by using in a combination with a reduction agent, such as sodiumsulfite, sodium hydrogen sulfite, ferrous sulfate, or L-ascorbic acid.The polymerization initiator used accounts preferably for 0.001 mol % to2 mol % inclusive, more preferably 0.01 mol % to 0.5 mol % inclusive, ofthe entire monomer.

The polymerized gel, suspended particles, and like materials can bedried using an ordinary drier or heating furnace. Alternatively, thematerials may be dried by azeotropic dehydration. The drier may be, forexample, a hot air drier, groove stirring drier, rotary drier, discdrier, flow layer drier, air flow drier, or infrared drier.

The materials are dried preferably at 100 to 250° C., more preferably at150 to 230° C., even more preferably at 160 to 210° C.

Solid content accounts for preferably 50 to 100 mass % (water content:50 to 0 mass %), more preferably 85 to 100 mass % (water content: 15 to0 mass %), even more preferably 90 to 98 mass % (water content: 10 to 2mass %), of the dried product obtained. The solid content rate isusually calculated from a weight loss in a 3-hour drying of a 1-gramsample in an aluminum cup or a glass petri dish at 180° C.

The dried product may be crushed/pulverized using, for example, avibration mill, roll granulator (see Japanese Unexamined PatentPublication 9-235378/1997 (Tokukaihei 9-235378), paragraph [0174]),knuckle pulverizer, roll mill (see Published Japanese Translation of PCTApplication (Tokuhyo) 2002-527547, paragraph [0069]), high speedrotation pulverizer (pin mill, hammer mill, screw mill, roll mill, etc.(see Japanese Unexamined Patent Publication 6-41319/1994 (Tokukaihei6-41319), paragraph [0036]), or cylindrical mixer (see JapaneseUnexamined Patent Publication 5-202199/1993 (Tokukaihei 5-202199),paragraph

Using an air flow drier or like machine, the product can be crushed anddried at the same time.

The water absorbing resin obtained by the drying andcrushing/pulverization preferably has a predetermined size (a narrowparticle size distribution) as a result of, for example, classificationbefore the resin is surface-crosslinked (detailed later). Preferably,the agglomerate of the water absorbing resin particles also has apredetermined size as a result of classification where necessary, likethe non-agglomerate water absorbing resin (detailed later).

The shape of the water absorbing resin particles obtained by thepolymerization explained above is typically, for instance, irregularlypulverized, spherical, fibrous, virgate, substantially spherical, orflat. It is preferred if the particles have an irregularly pulverizedshape because the particles can contain a large amount of silicondioxide (detailed later) at least either on the surface or near thesurface of the particles.

The water absorbing resin particles according to the present embodimentpreferably have regions near the surface crosslinked by an organicsurface crosslinking agent and/or a water-soluble inorganic surfacecrosslinking agent. When this is the case, the water absorbing resincontained in the water absorbing agent has regions near the surfacecrosslinked by a surface crosslinking agent. That reduces the amount ofliquid which may seep out when the swollen water absorbing agent isplaced under pressure. Absorption under the AAP pressure is thusimproved.

Examples of the surface crosslinking agent that can be used in thesurface crosslinking include organic surface crosslinking agents and/orwater-soluble inorganic surface crosslinking agents with two or morefunctional groups which can react with the functional groups,especially, carboxyl groups, of the water absorbing resin particles.Water-soluble organic surface crosslinking agents are preferred.

Examples include polyhydric alcohols, such as ethylene glycol,diethylene glycol, propylene glycol, triethylene glycol, tetraethyleneglycol, polyethylene glycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerine,polyglycerine, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexane dimethanol,1,2-cyclohexanol, trimethylolpropane, diethanol amine, triethanol amine,polyoxypropylene, oxyethylene-oxypropylene block copolymer,pentaerythritol, and sorbitol; epoxy compounds, such as ethylene glycoldiglycidyl ether, polyethylene glycol diglycidyl ether, glycerolpolyglycidyl ether, diglycerol polyglycidyl ether, polyglycerolpolyglycidyl ether, propylene glycol diglycidyl ether, polypropyleneglycol diglycidyl ether, and glycidol; polyvalent amine compounds, suchas ethylenediamine, diethylenetriamine, triethylenetetramine,tetraethylenepentamine, pentaethylenehexamine, and polyethyleneimine,and their inorganic and organic salts (for example, azetidinium salts);polyvalent isocyanate compounds, such as 2,4-tolylenediisocyanate andhexamethylenediisocyanate; polyvalent oxazoline compounds, such as1,2-ethylenebisoxazoline; derivatives of carbonic acids, such as urea,thiourea, guanidine, dicyandiamide, and 2-oxazolidinone; alkylenecarbonate compounds, such as 1,3-dioxolane-2-one,4-methyl-1,3-dioxolane-2-one, 4,5-dimethyl-1,3-dioxolane-2-one,4,4-dimethyl-1,3-dioxolane-2-one, 4-ethyl-1,3-dioxolane-2-one, 4-hydroxymethyl-1,3-dioxolane-2-one, 1,3-dioxane-2-one,4-methyl-1,3-dioxane-2-one, 4,6-dimethyl-1,3-dioxane-2-one, and1,3-dioxopane-2-one; haloepoxy compounds, such as epichlorohydrin,epibromohydrin, and α-methylepichlorohydrin, and their polyvalent amineadducts (for example, Kymene (Registered Trademark) manufactured byHercules Incorporated); silane coupling agents, such asγ-glycidoxypropyl trimethoxysilane and γ-aminopropyl triethoxysilane;and oxetane compounds, such as 3-methyl-3-oxetane methanol,3-ethyl-3-oxetane methanol, 3-butyl-3-oxetane methanol,3-methyl-3-oxetane ethanol, 3-ethyl-3-oxetane ethanol, 3-butyl-3-oxetaneethanol, 3-chloromethyl-3-methyl oxetane, 3-chloromethyl-3-ethyloxetane, and polyvalent oxetane compounds.

Any one of these surface crosslinking agents may be used alone;alternatively two or more of them may be used together. Among them,those surface crosslinking agents which have at least one hydroxyl groupare preferred. Especially, polyhydric alcohols are preferred becausethey are very safe and capable of improving the hydrophilic of the waterabsorbing resin particle surface.

The surface crosslinking agent used accounts preferably for 0.001 massparts to 5 mass parts, inclusive, of every 100 mass parts of the solidcontent of the water absorbing resin particles.

Water may be used in mixing the surface crosslinking agent with thewater absorbing resin particles. The water used accounts preferably for0.5 mass parts, exclusive, to 10 mass parts, inclusive, and morepreferably from 1 mass part to 5 mass parts, both inclusive, of every100 mass parts of the solid content of the water absorbing resinparticles.

A hydrophilic organic solvent or a third substance may be used as anauxiliary agent when mixing a surface crosslinking agent or its aqueoussolution with the water absorbing resin particles. Examples of suchhydrophilic organic solvents include lower alcohols, such as methylalcohol, ethyl alcohol, n-propyl alcohol, isopropyl alcohol, n-butylalcohol, isobutyl alcohol, and t-butyl alcohol; ketones, such asacetone; ethers, such as dioxane, tetrahydrofuran, andmethoxy(poly)ethylene glycol; amides, such as ε-caprolactam andN,N-dimethyl formamide; sulfoxides, such as dimethyl sulfoxide; andpolyhydric alcohols, such as ethylene glycol, diethylene glycol,propylene glycol, triethylene glycol, tetraethylene glycol, polyethyleneglycol, 1,3-propanediol, dipropylene glycol,2,2,4-trimethyl-1,3-pentanediol, polypropylene glycol, glycerine,polyglycerine, 2-butene-1,4-diol, 1,3-butanediol, 1,4-butanediol,1,5-pentanediol, 1,6-hexanediol, 1,2-cyclohexane dimethanol,1,2-cyclohexanol, trimethylolpropane, diethanol amine, triethanol amine,polyoxypropylene, oxyethylene-oxypropylene block copolymer,pentaerythritol, and sorbitol.

The hydrophilic organic solvent used may account preferably for 10 massparts or less, more preferably from 0 mass parts to 5 mass parts,inclusive, or from 0.1 mass parts to 5 mass parts, inclusive, of every100 mass parts of the solid content of the water absorbing resinparticles. That amount however may vary depending on the type, particlesize, and water content of the water absorbing resin particles, as wellas other factors.

The third substance may be, for instance, the inorganic, organic, orpolyamino acid described in the Specification of European Patent0668080. The auxiliary mixed agent may act as a surface crosslinkingagent, but preferably should not adversely affect the water absorptioncapability of the water absorbing resin particles after the surfacecrosslinking. The water absorbing resin particles of the presentembodiment is preferably crosslinked by mixing the particles with asurface crosslinking agent containing no hydrophilic organic solvent ofwhich the boiling point is 100° C. or below and then heating themixture. If the water absorbing resin particles contain a hydrophilicorganic solvent of which the boiling point is 100° C. or below, thehydrophilic organic solvent may vaporize, changing the environment inwhich the surface crosslinking agent resides on the surface of the waterabsorbing resin particles. One may not achieve sufficient SFC or otherphysical properties.

When the surface crosslinking agent is mixed with the water absorbingresin particles, preferably, a water-soluble inorganic salt (preferablya persulfate) is also present to obtain a more uniform mixture of thewater absorbing resin particles and the surface crosslinking agent. Thewater-soluble inorganic salt used accounts preferably for 0.01 massparts to 1 mass part, inclusive, more preferably 0.05 mass parts to 0.5mass parts, inclusive, of every 100 mass parts of the solid content ofthe water absorbing resin particles. That amount however may varydepending on the type and particle size of the water absorbing resinparticles, as well as other factors. In other words, the water absorbingresin particles of the present embodiment are preferably crosslinked bymixing the particles with an organic surface crosslinking agent and/or awater-soluble inorganic surface crosslinking agent containing awater-soluble inorganic salt (preferably a persulfate) in a ratio of0.01 mass % to 1.0 mass %, inclusive to the water absorbing resinparticles and then heating the mixture.

The method for mixing the surface crosslinking agent with the waterabsorbing resin particles is not limited in any particular manner. Forexample, the water absorbing resin particles may be immersed in ahydrophilic organic solvent and mixed with a surface crosslinking agentdissolved, as necessary, in water and/or a hydrophilic organic solvent.Another mixing method example may be to directly spray or add dropwiseto the water absorbing resin particles a surface crosslinking agentdissolved in water and/or a hydrophilic organic solvent.

After mixing the surface crosslinking agent with the water absorbingresin particles, heat is usually and preferably applied so that thecrosslink reaction can proceed. The heat treatment temperature, althoughvariable depending on the surface crosslinking agent being used, ispreferably from 40° C. to 250° C. inclusive, and more preferably from150° C. to 250° C. inclusive. If the heat treatment temperature is lowerthan 40° C., the AAP, SFC, and other absorption properties may not besufficiently improved. If the heat treatment temperature is higher than250° C., the excess heat may degrade the water absorbing resin particlesand hence various physical properties; care should be taken. The heattreatment time is preferably from 1 minute to 2 hours inclusive, andmore preferably from 5 minutes to 1 hour inclusive.

The water absorbing resin particles used in the present embodiment havea mass median particle size of, preferably, 200 μm to 500 μm inclusive,and more preferably 300 μm to 400 μm inclusive. If the water absorbingresin particles have a mass median particle size out of the 200 to 500μm range, the liquid permeability and diffusibility may fall noticeably,or the absorption rate may fall by a large value. Those water absorbingresin particles, if used in a diaper for example, may be leaky orotherwise defective.

Of the water absorbing resin particles of the present embodiment,preferably 5 mass % or less particles can pass through a sieve of 150-μmmesh, and more preferably 3 mass % or less particles can do so. The useof water absorbing resin particles in these ranges for the waterabsorbing agent limits the amount of dust in the resulting waterabsorbing agent. Thus, the fine particles in the water absorbing resinparticles will not fly off into the air, unlikely to raisesafety/hygiene issues, during manufacture of the water absorbing agent.Also, the physical properties of the resultant water absorbing agentwill not likely be degraded. If the ratio is in excess of 5 mass %, dustcan occur during manufacture of the water absorbing agent, possiblyraising safety/hygiene issues or degrading the physical properties ofthe water absorbent core, to name a few problems.

The water absorbing resin particles may be fine-powder-like waterabsorbing resin particles of which the mass median particle size is 300μm or less (hereinafter, referred to as “fine powder” where appropriate)which have been agglomerated, dried, adjusted in particle size, andsurface crosslinked. Alternatively, the water absorbing resin particlesmay be irregularly pulverized primary particles obtained frompulverization which have been partially mixed with an agglomerate of thefine powder. When an agglomerate of the fine powder is partially mixedwith the water absorbing resin particles, the resultant water absorbingagent has its absorption properties, such as water absorption rate andFHA, further improved. The amount of the agglomerate of the fine powdermixed with the water absorbing resin particles is preferably 5 mass % ormore, more preferably 10 mass % or more, further preferably, 15 mass %or more, most preferably 20 mass % or more. The particle size of finepowder is defined in terms of the mesh diameters of sieves used inclassification.

The agglomerate of the fine powder can be fabricated by any publiclyknown fine powder reproducing technology. For example, the fine powdermay be mixed with warm water and dried (Specification of U.S. Pat. No.6,228,930). The fine powder may be mixed with an aqueous solution of amonomer and polymerized (Specification of U.S. Pat. No. 5,264,495).Water may be added to the fine powder, and the mixture subjected toagglomeration under surface pressure more than a specific value(Specification of European Patent 844270). The fine powder may berendered sufficiently humid to form amorphous gel which is subsequentlydried and pulverized (Specification of U.S. Pat. No. 4,950,692). Thefine powder may be mixed with a polymerized gel (Specification of U.S.Pat. No. 5,478,879).

Preferably, the fine powder is mixed with warm water and dried as in oneof the methods briefly described above. The water absorbing resinparticles agglomerated by the method have a porous structure (equivalentto the porous structure described in Japanese Unexamined PatentPublication (Tokukai) 2004-261797) and is suited for use. The waterabsorbing resin particles of the present embodiment contain preferably 5mass % or more particles with the porous structure, more preferably 10mass % or more, further preferably 15 mass % or more, most preferably 20mass % or more. The inclusion of an agglomerate of fine powder with theporous structure in the water absorbing resin particles allows the waterabsorbing resin particles to efficiently contain silicon dioxide orother water-insoluble inorganic particles at least either on the surfaceor near the surface (detailed later).

The CRC of the water absorbing resin particles of the present embodimentis preferably 5 g/g or more, more preferably 10 g/g or more, furtherpreferably 15 g/g or more, even more preferably 25 g/g or more, stillpreferably 28 g/g or more, most preferably 30 g/g or more. The maximumvalue of the CRC, although not being limited in any particular manner,is preferably 50 g/g or less, more preferably 45 g/g or less, furtherpreferably 40 g/g or less. If the CRC is less than 10 g/g, and the waterabsorbing resin particles are used for the water absorbing agent, theagent can absorb a very small amount. The agent is not suitable for usein diaper and other sanitary/hygienic materials. On the other hand, ifthe CRC is more than 50 g/g, and the water absorbing resin particles areused in the water absorbent core, the resultant water absorbing agentmay fail to provide a water absorbent core with excellent liquid suctionrate per unit time.

The AAP of the water absorbing resin particles of the present embodimentis preferably 16 g/g or more, more preferably 17 g/g or more, even morepreferably 18 g/g or more, still more preferably 19 g/g or more, yetmore preferably 20 g/g or more, again more preferably 22 g/g or more,and most preferably 24 g/g or more. The maximum value of the AAP,although not being limited in any particular manner, is preferably 30g/g or less. If the AAP is less than 16 g/g, and the water absorbingresin particles are used for the water absorbing agent, the resultantwater absorbing agent may fail to achieve a low level of liquid seeping,or “rewetting,” when the water absorbent agent is placed under pressure.

The SFC of the water absorbing resin particles of the present embodimentis preferably 10 (10⁻⁷·cm³·s·g⁻¹) or more, more preferably 15(10⁻⁷·cm³·s·g⁻¹) or more, further preferably (10⁻⁷·cm³·s·g⁻¹) or more,even more preferably 50 (10⁻⁷·cm³·s·g⁻¹) or more, still more preferably70 (10⁻⁷·cm³·s·g⁻¹) or more, and yet most preferably 100(10⁻⁷·cm³·s·g⁻¹) or more. If the SFC is less than 10 (10⁻⁷·cm³·s·g⁻¹),even when the particles contain silicon dioxide or other water-insolubleinorganic particles (detailed later), liquid permeability may notimprove. Also, the resultant water absorbing agent may fail to provide awater absorbent core with excellent liquid suction rate per unit timewhen the water absorbing resin particles are used for the waterabsorbing agent.

The water-extractable polymer content of the water absorbing resinparticles of the present embodiment is preferably 35 mass % or less,more preferably 25 mass % or less, and further preferably 15 mass % orless. If the water-extractable polymer content exceeds 35 mass %, theparticles show poor gel strength and liquid permeability. If the waterabsorbing resin particles are used in the water absorbent core, theresultant water absorbing agent may fail to achieve a low level ofliquid seeping, or “rewetting,” when the water absorbent core is placedunder pressure.

Water-Insoluble Inorganic Particles

The water absorbing resin particles of the present embodiment containswater-insoluble inorganic particles at least either on the surface ornear the surface. The “surface” of a water absorbing resin particlerefers to those portions which are exposed to air. The term “near thesurface” of the water absorbing resin particle refers to those portionsbetween the surface of the particle and a depth about one tenth of theparticle size (smaller size). Specifically, the term “near the surface”of the water absorbing resin particle refers to those portions near asurface-crosslinked layer in the case of a surface-crosslinked waterabsorbing resin particle. The thickness of the layer can be confirmedwith, for example, a scanning electron microscope (SEM).

The provision of the water-insoluble inorganic particles at least eitheron the surface or near the surface of the water absorbing resinparticles results in improvement in the liquid permeability of the waterabsorbing agent. Therefore, the SFC of the water absorbing agentcontaining the water absorbing resin particles is improved. It would besufficient for the purpose of liquid permeability improvement if thewater-insoluble inorganic particles reside at least either on thesurface or near the surface of the water absorbing resin particles.Physical properties of the water absorbing agent are better improved ifthe particles reside on the surface than near the surface. In theexamples detailed later, the water-insoluble inorganic particles resideat least on the surface. Also, the ratio of the water-insolubleinorganic particles residing on the surface is increased by adding thewater-insoluble inorganic particles after polymerization.

The water-insoluble inorganic particles used in the present embodimentpreferably contain, at least on the surface, functional groups capableof forming ionic bonds with functional groups on the surface of thewater absorbing resin particles. The functional groups capable offorming ionic bonds are more preferably cationic groups, even morepreferably amino groups (inclusive quaternary amino groups).

In a preferable form of the water absorbing agent, the functional groupsresiding on the surface of the water absorbing resin particles arecarboxyl groups, and the functional groups residing at least on thesurface of the water-insoluble inorganic particles are amino groups.

Concrete examples of the water-insoluble inorganic particles which canbe used in the present embodiment include mineral produces, such astalc, clay, kaoline, fuller's earth, bentonite, activated clay, barite,natural asphaltum, strontium ore, ilmenite, and pearlite; metal oxides,such as silicon dioxide and titanium oxide; silicic acids (salts), suchas natural zeolite and synthetic zeolite; water-insoluble polyvalentmetal salts, such as calcium sulfate and aluminum oxide; hydrophilicamorphous silica (ex. Dry Method: ReolosilQS-20 from TokuyamaCorporation. Precipitation Method: Sipernat 22S, Sipernat 2200 fromDegussa AG); composite, water-containing hydrated oxides containingeither zinc and silicon and or zinc and aluminum (see InternationalApplication Published under PCT WO2005/010102 for examples); and oxidecomplexes, such as silicon oxide/aluminum oxide/magnesium oxidecomplexes (ex. Attagel #50 from ENGELHARD), silicon oxide/aluminum oxidecomplexes, and silicon oxide/magnesium oxide complexes. Thewater-insoluble inorganic particles disclosed also in U.S. Pat. No.5,164,459 and European Patent 761241 may also be used. Preferred amongthese examples are silicon dioxide and silicic acid (salt). Especiallypreferred are silicon dioxide and silicic acid (salt) in fine particleform of which the mean particle size is from 0.001 to 200 μm as measuredby a Coulter counter method.

Some concrete examples of the water-insoluble inorganic particles mostpreferably used in the present embodiment are HDK (Registered Trademark)H2015EP, H2050EP, H2150VP, H05TA, H13TA, and H30TA, from Wacker whichare amorphous silica with amino groups (inclusive quaternary aminogroups) introduced to its surface. RA200HS from Nippon Aerosil Co., Ltd.may also be used.

The water-insoluble inorganic particles used in the present embodimentpreferably have a pH of 7 to 10 when the particles are dispersed in awater-methanol solution (1:1 in volume ratio) in the amount of 4 mass %.

The water-insoluble inorganic particles used in the present embodimentpreferably have a mass median particle size of 5 to 50 nm in primaryparticle form and are such that 90 mass % or more of the particles areaggregate particles of primary particles. The aggregate particles ofprimary particles preferably have a mass median particle size of 20 μmor less.

The water-insoluble inorganic particles used in the present embodimentpreferably have 20% or less of residual silanol groups on the surface(the silanol groups on the surface account for 2 SiOH/nm² of the wholesilanol groups).

The water-insoluble inorganic particles used in the present embodimentpreferably has a specific surface area of 30 to 330 m²/g as measured byBET.

Silicon Dioxide

As mentioned earlier, in the present embodiment, the water-insolubleinorganic particles preferably contain silicon dioxide.

The provision of silicon dioxide at least either on the surface or nearthe surface of the water absorbing resin particles results inimprovement in the liquid permeability of the water absorbing agent.Therefore, the SFC of the water absorbing agent containing the waterabsorbing resin particles is improved. It would be sufficient for thepurpose of liquid permeability improvement if the silicon dioxideresides at least either on the surface or near the surface of the waterabsorbing resin particles. Physical properties of the water absorbingagent are better improved if the silicon dioxide resides on the surfacethan near the surface.

The silicon dioxide is preferably amorphous fumed silica manufactured bya dry method. “Quartz” and like silicon dioxides are not desirable dueto health risks they may pose. The silicon dioxide preferably does notundergo self-induced aggregation and contains a high concentration ofresidual silanol groups. Self-induced aggregation of the silicon dioxideis the cause of increase in the amount of dust produced from the waterabsorbing agent. The present embodiment uses the silicon dioxide in aparticular range of amount, and the silicon dioxide contains residualsilanol groups in a particular range of concentration, to preventself-induced aggregation of the silicon dioxide and thereby preventincrease in the amount of dust produced from the water absorbing agent.

The concentration of residual silanol groups on the surface of thesilicon dioxide is given in terms of the number of silanol groups in onesquare nanometer (nm²). In the present invention, a unit “SiOH/nm²” isused. One can consult a catalogue or like documents available fromsilica manufacturers to find out the residual silanol groupconcentration. Alternatively, it can be measured by a publicly knownmethods (ex. a lithium aluminum hydride method).

In the present embodiment, the residual silanol group concentration onthe surface of the silicon dioxide is preferably 1.7 SiOH/nm² or lower.At such low residual silanol group densities, the silicon dioxide ismodified at the surface and thus hydrophobic. If the concentrationexceeds 1.7 SiOH/nm², the silicon dioxide is likely to aggregate andthus produce dust, which is undesirable. The surface modification of thesilicon dioxide hinders the silanol groups from forming hydrogen bondsbetween them, and hence the silicon dioxide in fine particle form fromaggregating. The silicon dioxide content is as low as from 10 ppm to1,900 ppm inclusive to the water absorbing resin particles. With suchlow non-aggregated silicon dioxide content, its fine particles areadsorbed by interparticle bonding force, such as electrostatic force andvan der Waals force, to the water absorbing resin particles. Therefore,the amount of dust produced because of the provision of the silicondioxide in the water absorbing resin particles is limited.

If the silicon dioxide of which the residual silanol group concentrationexceeds 1.7 SiOH/nm² is used, there reside many silanol groups on thesurface of the silicon dioxide; silanol groups form hydrogen bondsbetween them, and the fine particles of the silicon dioxide aggregate.The aggregated fine particles are not well adsorbed by the waterabsorbing resin particles and come off easily. The fine particles thusfly away forming dust. It is difficult to limit the amount of dust.

The concentration of the residual silanol groups on the surface of thesilicon dioxide is preferably 1.7 SiOH/nm² or lower, more preferablyfrom 0.7 SiOH/nm² to 1.7 SiOH/nm² inclusive, further preferably from 0.9SiOH/nm² to 1.7 SiOH/nm². If the concentration is lower than 0.7SiOH/nm², the silicon dioxide becomes more hydrophobic, and LDV falls,which is not desirable.

Concrete examples of the silicon dioxide contained in the waterabsorbing resin particles include HDK (Registered Trademark) H15 (≈0.96SiOH/nm²), H20 (≈1.00 SiOH/nm²), H30 (≈1.08 SiOH/nm²), H1303VP (≈0.36SiOH/nm²), H2000/4 (≈0.60 SiOH/nm²), H2000T (≈0.36 SiOH/nm²), and H3004(≈0.40 SiOH/nm²), all available from Wacker. Examples of the silicondioxide include H05TD, H13TD, H20TD, H30TD, H05TM, H13TM, H20TM, H30TM,H05TX, H13TX, H20TX, and H30TX, all having a residual silanol groupconcentration of about 0.40 SiOH/nm² or lower. Other examples includeAerosil (Registered Trademark) R-972 (≈0.60 SiOH/nm²), R-974 (≈0.39SiOH/nm²), R805 (≈1.66 SiOH/nm²), R812 (≈0.44 SiOH/nm²), R812S (≈0.68SiOH/nm²), and R202 (≈0.29 SiOH/nm²), all manufactured by Nippon AerosilCo., Ltd., Reolosil (Registered Trademark) MT-10 (C), DM-10 (C), DM-30,DM-30S, KS-20SC, HM-20L, HM-30S, and PM-20 (L), all manufactured byTokuyama Corporation.

Water Absorbing Agent

The water absorbing agent of the present embodiment contains waterabsorbing resin particles with an internal crosslinking structureobtained by polymerization of a water-soluble unsaturated monomer. Theagent satisfies conditions (a) to (d) below:

(a) the agent contains water-insoluble inorganic particles at an amountof from 10 ppm to 1,900 ppm inclusive;

(b) the agent contains 5 mass % or less particles which have such a sizethat they can pass through a sieve having a mesh opening size of 150 μm;

(c) the agent has an absorbency against a pressure of 4.83 kPa (AAP) of18 g/g or more; and

(d) the water-insoluble inorganic particles reside on the surface of thewater absorbing resin or near the surface.

The water absorbing agent of the present embodiment containswater-insoluble inorganic particles at an amount of from 10 ppm to 1,900ppm inclusive, more preferably 10 to 1,500 ppm, most preferably 10 to990 ppm, relative to the water absorbing agent. If there are containedtoo many water-insoluble inorganic particles, safety/health concerns mayarise due to fine particles flying away during the manufacture of theabsorbent core, and the performance of the absorbent core be degraded.If too many water-insoluble inorganic particles are used for theabsorbent core, the resultant water absorbing agent may be such that theabsorbent core does not show sufficient vertical liquid suctioncapability (i.e., fixed height absorbency (FHA)).

In the water absorbing agent of the present embodiment, the silicondioxide detailed above (with a residual silanol group concentration of1.7 SiOH/nm² or lower on the surface) may be replaced with silicondioxide which has yet to undergo self-induced aggregation or of whichthe self-induced aggregation has been broken by, for example, mechanicalforce. The use of the silicon dioxide with a residual silanol groupconcentration of 1.7 SiOH/nm² or lower is merely one of means oflimiting self-induced aggregation of the silicon dioxide. Whether thesilicon dioxide in the water absorbing resin particles has or has notundergone self-induced aggregation can be determined by analyzing fineparticles filtered out after the measurement of dust amount (detailedlater). For example, the filtered-out fine particles are observed undera scanning electron microscope or an X-ray microanalyzer; if silicondioxide particles with major diameters of 20 μm to 100 μm, inclusive,are found among the fine particles, and the ratio of the number of thosesilicon dioxide particles to the number of filtered-out fine particleswith major diameters of 20 μm to 100 μm, inclusive, is 10% or higher,the silicon dioxide is determined to have undergone self-inducedaggregation.

In other words, the water absorbing agent of the present embodimentcontains water absorbing resin particles with an internal crosslinkingstructure obtained by polymerization of a water-soluble unsaturatedmonomer. In the agent, the water absorbing resin particles arecrosslinked by a surface crosslinking agent near the surface and containsilicon dioxide at least either on the surface or near the surface.Furthermore, of the fine particles with major diameters of 20 μm to 100μm, inclusive, filtered out after the measurement of dust amount in thewater absorbing agent, 10% or less is accounted for by silicon dioxide.Besides, the silicon dioxide content of the water absorbing agent is 10ppm to 1,900 ppm inclusive. The mass median particle size of the waterabsorbing agent is from 200 μm to 500 μm inclusive. Particles which passthrough a sieve having a mesh opening size of 150 μm constitute 5 mass %or less of the mass of the entire water absorbing agent.

The water absorbing agent of the present embodiment preferably containsat least 0.001 mass % to 5 mass % inclusive, more preferably 0.01 mass %to 1 mass % inclusive, polyvalent metal salt. The abundant provision ofpolyvalent metal salt in the water absorbing agent (preferably trivalentwater-soluble polyvalent metal salt) improves saline flow conductivitywhile substantially preserving the absorbency under 4.83 kPa load andfixed height absorbency of the water absorbing agent. The provision alsomakes up for the lack of the silicon dioxide or other water-insolubleinorganic particles at least either on the surface or near the surfaceof the water absorbing agent where there is no silicon dioxide or otherwater-insoluble inorganic particles. The obtained water absorbing agenthas more excellent physical properties because of the synergisticeffects of the silicon dioxide or other water-insoluble inorganicparticles and the polyvalent metal salt.

Concrete examples of the polyvalent metal salt that can be used in thepresent embodiment include sulfates, nitrates, carbonates, phosphates,organic acid salts, and halides (ex. chlorides) of Zn, Be, Mg, Ca, Sr,Al, Fe, Mn, Ti, Zr, Ce, Ru, Y, Cr, and like metals. Another example isthe polyvalent metal salt described in Japanese Unexamined PatentPublication (Tokukai) 2005-113117.

Most preferred of the polyvalent metal salt are trivalent water-solublemetal salts. Concrete examples of trivalent water-soluble polyvalentmetal salts include aluminum chloride, aluminum polychloride, aluminumsulfate, aluminum nitrate, aluminum potassium sulfate, aluminum sodiumsulfate, potassium alum, ammonium alum, sodium alum, sodium aluminate,iron(III) chloride, cerium(III) chloride, ruthenium(III) chloride,yttrium(III) chloride, and chromium(III) chloride.

It is preferable to use these salts which contain crystal water also inview of the solubility of urine and other liquids absorbed. Especiallypreferred among them are aluminum compounds, especially, aluminumchloride, aluminum polychloride, aluminum sulfate, aluminum nitrate,aluminum potassium sulfate, aluminum sodium sulfate, potassium alum,ammonium alum, sodium alum, and sodium aluminate. Aluminum sulfate isparticularly preferred. The most preferred is an aqueous solution ofaluminum sulfate (desirably, a solution of aluminum sulfate with a 90%or higher concentration as based on saturation). Any one of thesecompounds may be used alone; alternatively two or more of them may beused together. One of the most preferred forms of the water absorbingagent of the present embodiment is the water absorbing agent thatcontains silicon dioxide and a trivalent water-soluble polyvalent metalsalt.

The water absorbing agent of the present embodiment produces littledust. Dust, if present at all, is preferably 400 ppm or less, morepreferably 355 ppm or less, further preferably 300 ppm or less, stillmore preferably 240 ppm or less, as measured with a Heubach Dustmeter(detailed later). So long as one of these conditions is met, silicondioxide or other water-insoluble inorganic particles, if contained inthe water absorbing agent, will not spread into the air, unlikely toraise safety/hygiene issues, during manufacture of the water absorbingagent. Also, the physical properties of the water absorbent core willnot likely be degraded.

The water absorbing agent has a mass median particle size of,preferably, 200 μm to 500 μm, inclusive, more preferably 300 μm to 400μm, inclusive. If the mass median particle size is out of this 200 μm to500 μm range, the liquid permeability may fall, and the liquid suctionrate per unit time of the water absorbing agent may fall noticeably. Inother words, the absorption rate may fall by a large value, and thatabsorbing agent, if used in a diaper for example, may be leaky orotherwise defective.

Particles that can pass through a sieve of 150-μm mesh preferablyconstitute 5 mass % or less of the water absorbing agent. The ratio ismore preferably 4 mass % or less, and further preferably 3 mass % orless. If the ratio is in excess of mass %, even when the silicon dioxideor other water-insoluble inorganic particles of the present embodimentare contained, particles can fly away during manufacture of the waterabsorbing agent, possibly raising safety/hygiene issues or degrading thephysical properties of the water absorbent core obtained.

The particle size distribution of the water absorbing agent has alogarithmic standard deviation (σζ) of preferably 0.20 to 0.50,inclusive, more preferably 0.20 to 0.40, inclusive, and even morepreferably 0.30 to 0.40, inclusive. If the standard deviation is out ofthese ranges, the liquid permeability may so decrease that the waterabsorbent core has a very poor liquid suction rate per unit time.

The water absorbing agent has a CRC of preferably 5 g/g or more, morepreferably 10 g/g or more, even more preferably 15 g/g or more, stillmore preferably 25 g/g or more, and yet preferably 28 g/g or more.

The maximum CRC, although not limited in any particular manner, ispreferably 50 g/g or less, more preferably 45 g/g less, and even morepreferably 40 g/g or less. If the CRC is less than 5 g/g, the waterabsorbing agent absorbs too small an amount of liquid to be used asdiapers and other sanitary/hygienic materials. If the centrifugeretention capacity (CRC) is more than 50 g/g, and the obtained waterabsorbing agent is used in the water absorbent core, the core may notexhibit an excellent liquid suction rate per unit time into the waterabsorbent core.

The water absorbing agent of the present embodiment has a SFC ofpreferably 10 (10⁻⁷·cm³·s·g⁻¹) or more, more preferably 15(10⁻⁷·cm³·s·g⁻¹) or more, even more preferably 30 (10⁻⁷·cm³·s·g⁻¹) ormore, still more preferably 50 (10⁻⁷·cm³·s·g⁻¹) or more, yet morepreferably 70 (10⁻⁷·cm³·s·g⁻¹) or more, most preferably 100(10⁻⁷·cm³·s·g⁻¹) or more. If the SFC is less than 10 (10⁻⁷·cm³·s·g⁻¹),even when the silicon dioxide or other water-insoluble inorganicparticles are added, the liquid permeability does not improve. The waterabsorbing agent obtained, when used in the water absorbent core, doesnot exhibit an excellent liquid suction rate per unit time to the waterabsorbent core. The maximum SFC, although not limited in any particularmanner, is preferably 2,000 (10⁻⁷·cm³·s·g⁻¹) or less.

The water absorbing agent of the present embodiment preferably haswell-balanced CRC and SFC. Specifically, if the CRC is 5 g/g or more andless than 25 g/g, the SFC is preferably 100 (10⁻⁷·cm³·s·g⁻¹) or more,more preferably 150 (10⁻⁷·cm³·s·g⁻¹) or more, and even most preferably300 (10⁻⁷·cm³·s·g⁻¹) or more. If the CRC is 25 g/g or more and less than30 g/g, the SFC is preferably 30 (10⁻⁷·cm³·s·g⁻¹) or more, morepreferably 70 (10⁻⁷·cm³·s·g⁻¹) or more, most preferably 100(10⁻⁷·cm³·s·g⁻¹) or more. If the CRC is from 30 g/g inclusive to 50 g/gexclusive, the SFC is preferably 10 (10⁻⁷·cm³·s·g⁻¹) or more, morepreferably 15 (10⁻⁷·cm³·s·g⁻¹) or more, even more preferably 30(10⁻⁷·cm³·s·g⁻¹) or more, most preferably 50 (10⁻⁷·cm³·s·g⁻¹).

If the CRC and SFC are within one of these ranges, the water absorbingagent, when used in the water absorbent core, shows a sufficiently highabsorption which makes up for possibly liquid permeability. On the otherhand, if the liquid permeability is high, liquid diffuses in the waterabsorbing agent, enabling absorption across a wide area, even when theagent absorbs a small amount of liquid. Thus, the resulting waterabsorbing agent exhibits an excellent liquid suction rate per unit timewhen used in the water absorbent core.

The water absorbing agent has an absorbency against a pressure of 4.83kPa (AAP) of 18 g/g or more. The value is more preferably 20 g/g ormore, even more preferably 22 g/g or more, and most preferably 24 g/g ormore. The maximum AAP, although not limited in any particular manner, ispreferably 30 g/g or lower. If the absorbency against a pressure of 4.83kPa (AAP) is lower than 18 g/g, the water absorbing agent, when used inthe water absorbent core, may cause lot of liquid seeping, or“rewetting,” when the water absorbent core is placed under pressure.

Of the entire water absorbing agent, those particles of which theparticle sizes are from 150 to 850 μm are preferably 90 mass % or more.More preferably, those particles of which the particle sizes are from150 to 600 μm are 90 mass % or more.

The water-extractable polymer content of the water absorbing agent ispreferably 35 mass % or less, more preferably 25 mass % or less, evenmore preferably 15 mass % or less. If the water-extractable polymercontent of the water absorbing agent exceeds 35 mass %, the gel showspoor strength and liquid permeability. Besides, if the water absorbingagent is used in a diaper over an extended period of time, the CRC, AAP,etc. may be degraded over time.

The water absorbing agent is preferably charged negative afterexperiencing frictional motion. That prevents aggregation of the silicondioxide and the water absorbing agent and restrains the silicon dioxidefrom coming off the water absorbing resin particles. As a result, theamount of dust of the water absorbing agent is decreased.

The water absorbing agent has a liquid distribution velocity ofpreferably 0.2 mm/sec to 10.0 mm/sec inclusive, more preferably 0.5mm/sec to 10.0 mm/sec inclusive, even more preferably 0.8 mm/sec to 10.0mm/sec inclusive.

Accordingly, the liquid absorbed by the water absorbing agent diffusesefficiently. That improves the liquid suction rate per unit time of thewater absorbent core and liquid diffusibility in the water absorbentcore. Thus, water absorption capability is improved.

A plant component A1, chelate agent B1, other substance C1, etc.(detailed later) may be added as in a very small amount to the waterabsorbing agent to impart the particulate water absorbing agent withvarious functions.

The amount of the additives A1 to C1 used may vary depending on objectsand additional functions. Generally, one of such additives may be addedin 0 to 10 mass parts, preferably 0.001 to 5 mass parts, more preferably0.002 to 3 mass parts, to 100 mass parts of the water absorbing resin.Generally, if the amount is less than 0.001 mass parts, sufficienteffects or addition functions are not achieved. If the amount is 10 massparts or more, effects cannot be achieved in proportion to the amountadded, or the absorbency may fall.

Plant Component A1

The particulate water absorbing agent of the present embodiment may beblended with a plant component so that the agent can be deodorizing. Thepreferred ratio of the plant component blended is as mentioned above.Plant components that may be used in the present embodiment arepreferably a powder of a plant itself or an extract of the plant. Thecompound(s) in the plant component is/are preferably either at least onecompound selected from polyphenol, flavone and like substances, andcaffeine or at least one compound selected from tannin, tannic acid,galla, gallnut, and gallic acid. Examples are found in U.S. Pat. No.6,469,080, European Patent 1352927, and International ApplicationPublished under PCT WO2003/104349, as examples. Examples of the form inwhich the plant component is blended in the present invention includeessence (essential oil, etc.) extracted from the plant, a plant per se(plant powder, etc.), plant refuse and extraction refuse which arebyproducts of manufacturing processes in the plant processing industryand the food processing industry.

The particle size of powder when the plant component A1 is provided inpowder form and/or the particle size of powder which carries essence(essential oil) containing the plant component A1 extracted from a plantis generally from 0.001 to 1,000 μm, preferably from 1 to 600 μm. Themass median particle size of the particles containing a plant componentis preferably 500 μm or less, more preferably 300 μm or less. If themass median particle size is more than 500 μm, the effective componentin the plant component, when coming into contact with urine, does notwork sufficiently, possibly failing to achieve stable deodorizingcapability. The mass median particle size is preferably smaller than themass median particle size of the water absorbing resin to achieveexcellent deodorizing capability and stability. Examples of particlescontaining a plant component include a plant powder into which a plantper se has been made into and a particulate carrier carrying a plantcomponent. Examples of such a carrier include those carrying essence(essential oil) containing a plant component extracted from a plant. Theplant component usable in the present embodiment preferably takes theform of liquid and/or aqueous solution at normal temperature so that theplant component can be readily added to the water absorbing resin.

Addition of Chelate Agent B1

A chelate agent, especially a polyvalent carboxyl acid and its salt, ispreferably blended to obtain the particulate water absorbing agent ofthe present embodiment.

The chelate agent that can be used for the particulate water absorbingagent of the present embodiment is preferably a chelate agent with ahigh Fe or Cu sequestering or chelating capability: namely, a chelateagent with a stability constant with respect to Fe ions of 10 or more,preferably 20 or more, more preferably an amino polyvalent carboxyl acidand its salt, even more preferably an amino carboxyl acid with three ormore carboxyl groups and its salt.

The polyvalent carboxyl acids are, specifically,diethylenetriaminepentaacetic acid, triethylenetetraaminehexaaceticacid, cyclohexane-1,2-diaminetetraacetic acid, N-hydroxyethylethylenediaminetriacetic acid, ethylene glycol diethyl etherdiaminetetraacetic acid, ethylenediamine tetrapropionic acetic acid,N-alkyl-N′-carboxy methyl aspartic acid, N-alkenyl-N′-carboxy methylaspartic acid, and their alkali metal salts, alkali earth metal salts,ammonium salts, and amine salts. The most preferred among them arediethylenetriaminepentaacetic acid, triethylenetetraaminehexaaceticacid, N-hydroxyethyl ethylenediaminetriacetic acid, and their salts.

Other Substances C1

Various additives, such as an antibacterial agent, a water-solublepolymer, a water-insoluble polymer, water, a surfactant, organic fineparticles, may or may not be added provided that the particulate waterabsorbing agent of the present embodiment is obtainable.

The water absorbing agent of the present embodiment contains waterabsorbing resin particles which may or may not contain a polyvalentmetal salt and water-insoluble inorganic particles. Specifically, thewater absorbing agent of the present embodiment may contain waterabsorbing resin particles obtained by polymerization of a water-solubleunsaturated monomer. The agent satisfies conditions (A) to

-   -   (D) below:

(A) the particles are, near the surface, either crosslinked or coatedwith a surface crosslinking agent which has at least one hydroxyl group;

(B) the particles contain a polyvalent metal salt and water-insolubleinorganic particles at least either on the surface or near the surface;

(C) the water absorbing agent has a mass median particle size of 200 μmto 500 μm inclusive; and

(D) the water absorbing agent contains 5 mass % or less particles whichhave such a size that they can pass through a sieve having a meshopening size of 150 μm.

The water absorbing resin particles in the water absorbing agent iseither crosslinked or coated near the surface with a surfacecrosslinking agent which has at least one hydroxyl group. The waterabsorbing resin contained in the water absorbing agent is eithercrosslinked or coated near the surface with a surface crosslinking agentwhich has at least one hydroxyl group as stated above. That reduces theamount of liquid which may seep out when the swollen water absorbingagent is placed under pressure. Therefore, the absorbency againstpressure, or AAP, is increased.

The water absorbing agent is not limited by any particular means exceptthe condition that the water absorbing agent contains water absorbingresin particles obtained by polymerization of a water-solubleunsaturated monomer and items (A) to (D). For example, the waterabsorbing agent may be conditioned within the aforementioned rangesdescribed in relation to the water absorbing agent meeting conditions(a) to (d). Also, conditions (A) to (D) may be narrowed down to theranges described in relation to the water absorbing agent meetingconditions (a) to (d), to impart excellent physical properties to thewater absorbing agent.

The water absorbing agent that meets conditions (A) to (D) preferablycontains water-insoluble inorganic particles. The ratio of the particlesto the water absorbing agent is preferably from 0.001 mass % to 5 mass %inclusive, more preferably from 0.01 mass % to 3 mass % inclusive, evenmore preferably from 0.01 mass % to 1 mass % inclusive, still morepreferably from 0.01 mass % to 0.4 mass % inclusive, most preferablyfrom 0.05 mass % to 0.4 mass % inclusive. The upper and lower limits ofthe numeric range may be combined appropriately. A preferred numericvalue range, as an example, is from 0.001 mass % to 0.4 mass %inclusive. Water-insoluble inorganic particles are contained at leasteither on the surface or near the surface of the water absorbing resinparticles as stated above. That improves saline flow conductivity andpowder fluidity at high humidity. In addition, the aforementionedpolyvalent metal salt also resides at least either on the surface ornear the surface of the water absorbing resin particles. The synergisticeffects of the water-insoluble inorganic particles and the polyvalentmetal salt further enhances the physical properties of the resultantwater absorbing agent.

The water absorbing agent has an absorbency against a pressure of 4.83kPa (AAP) of preferably 16 g/g or more. The maximum value, although notlimited in any particular manner, is preferably 30 g/g or less. If theabsorbency against a pressure of 4.83 kPa (AAP) is less than 16 g/g, theresultant water absorbing agent, when used in the water absorbent core,has a relatively high level of liquid seeping, or “rewetting,” when thewater absorbent core is placed under pressure.

Manufacturing Method for Water Absorbing Agent

The method of manufacturing a water absorbing agent of the presentembodiment (manufacturing method A) is a method of manufacturing a waterabsorbing agent containing water absorbing resin particles which has aninternal crosslinking structure obtained by polymerization of awater-soluble unsaturated monomer. In the method, the water absorbingresin particles are mixed with water-insoluble inorganic particles whilethe resin particles are being surface crosslinked or before or afterthat crosslinking. The resin particles are crosslinked near the surfacewith an organic surface crosslinking agent and/or a water-solubleinorganic surface crosslinking agent. The resin particles have a massmedian particle size of 200 to 500 μm. The resin particles contain 5mass % or less particles which have such a size that they can passthrough a sieve having a mesh opening size of 150 μm. The inorganicparticles have at least on the surface thereof functional groups whichare capable of forming ionic bonds with functional groups on the surfaceof the water absorbing resin particles.

The water absorbing resin particles used in the method of manufacturinga water absorbing agent of the present embodiment are preferably theaforementioned water absorbing resin particles. In addition, thewater-insoluble inorganic particles used in the method of manufacturinga water absorbing agent of the present embodiment are preferably theaforementioned water-insoluble inorganic particles.

In the method of manufacturing a water absorbing agent of the presentembodiment, the water-insoluble inorganic particles are added in anamount of 1 to 10,000 ppm, more preferably 5 to 1,500 ppm, mostpreferably 10 to 990 ppm, to the water absorbing resin particles. Ifthere are contained too many water-insoluble inorganic particles, fineparticles may fly away during the manufacture of an absorbent core,possibly raising safety/hygiene issues or inducing degradation of theperformance of the absorbent core. In addition, the resultant agent,when used in an absorbent core, may be such that the absorbent core doesno show excellent vertical liquid suction capability, (i.e., fixedheight absorbency (FHA)).

The most preferred form of the method of manufacturing a water absorbingagent involves a step of mixing an at least trivalent water-solublepolyvalent metal salt as a solution (preferably, aqueous solution) withthe water absorbing resin particles and a step of mixing thewater-insoluble inorganic particles.

The water absorbing agent of the present embodiment may be manufacturedby a method of manufacturing a water absorbing agent, with an internalcrosslinking structure, which contains water absorbing resin particlesobtained by polymerization of a water-soluble unsaturated monomer. Thewater absorbing resin particles are crosslinked near the surface with asurface crosslinking agent. The water absorbing resin particles have amass median particle size of 200 μm to 500 μm inclusive. Silicon dioxideof which the residual silanol group concentration is 1.7 SiOH/nm² orlower is mixed with the water absorbing resin particles in an amount of10 ppm to 1,900 ppm inclusive based on the resin particles at least at atime selected from the group consisting of while being crosslinked withthe surface crosslinking agent, before that crosslinking, and after thatcrosslinking. This method is called manufacturing method B.

In the method of manufacturing a water absorbing agent of the presentembodiment (manufacturing method B), the water absorbing resin particlesare first crosslinked near the surface with a surface crosslinkingagent. The surface crosslinking agent and method mentioned under theheading “Water Absorbing Resin Particles” may be employed.

The timing of mixing the silicon dioxide or other water-insolubleinorganic particles and the water absorbing resin particles may be atleast a time selected from the group consisting of while beingcrosslinked with the surface crosslinking agent, before thecrosslinking, or after the crosslinking. The timing is not limited inany particular manner. The timing is preferably after the crosslinking,more preferably after crosslinking and when the particles aremechanically damaged.

The specific method of mixing the silicon dioxide or otherwater-insoluble inorganic particles with the water absorbing resinparticles in manufacturing methods A, B may use any publicly knownstirring device: to name a few examples, the paddle blender, the ribbonmixer, the rotary blender, the jar tumbler, the browser mixer, themortar mixer. These stirring devices may include a heating device whichheats the mixture of the water absorbing resin particles and thewater-insoluble inorganic particles and also a cooling device whichcools down the mixture heated up by the heating device.

Stirring time, although not limited in any particular manner, ispreferably 60 minutes or shorter, more preferably 30 minutes or shorter.

It is important to prevent the water absorbing resin particles and thesilicon dioxide or other water-insoluble inorganic particles fromaggregating when they are mixed, because they are particulate powder.Therefore, the water-insoluble inorganic particles and the waterabsorbing resin particles are preferably transported pneumatically whenor after the water-insoluble inorganic particles and the water absorbingresin particles are mixed. The pneumatic transportation prevents thewater absorbing resin particles and the silicon dioxide or otherwater-insoluble inorganic particles from aggregating. Therefore, thesilicon dioxide or other water-insoluble inorganic particles can beuniformly mixed with the water absorbing resin particles. The physicalproperties of the water absorbing agent obtained are enhanced.

In the manufacturing methods (manufacturing methods A, B), thepolyvalent metal salt and the water absorbing resin particles are mixedpreferably at least while being crosslinked with the surfacecrosslinking agent, before that crosslinking, or after thatcrosslinking, so as to manufacture a water absorbing agent withexcellently balanced centrifuge retention capacity and saline flowconductivity.

The amount of the polyvalent metal salt used is preferably from 0.001mass % to 5 mass % inclusive, more preferably from 0.01 mass % to 1 mass% inclusive, based on the water absorbing resin particles.

When the polyvalent metal salt is mixed, the salt is preferably mixed asan aqueous solution. The concentration of the water-soluble polyvalentmetal salt in the aqueous solution containing the polyvalent metal saltis preferably 50% or more, more preferably 60% or more, even morepreferably 70% or more, still more preferably 80% or more, yet morepreferably 90% or more, relative to the saturation concentration, so asto prevent infiltration and diffusion into the water absorbing resinparticles. The salt may of course be used at the saturationconcentration. In addition, the aforementioned hydrophilic organicsolvent and an organic acid (or salt thereof), such as lactic acid (orsalt thereof), may be present together in an aqueous solution containingat least the polyvalent metal salt. The co-presence of the hydrophilicorganic solvent and an organic acid is preferred because, at least theinfiltration and diffusion of the polyvalent metal salt into the waterabsorbing resin particles are limited, and the salt is better mixed.

The stirring device which mixes the polyvalent metal salt, although notlimited in any particular manner, may be one of the aforementionedstirring devices, used with the water-insoluble inorganic particles.Stirring time may also be specified similarly to the case of theaforementioned water-insoluble inorganic particles.

In the method of manufacturing a water absorbing agent, the silicondioxide or other water-insoluble inorganic particles and the waterabsorbing resin particles are mixed preferably after the water absorbingresin particles are mechanically damaged, so as to irregularly pulverizethe water absorbing resin particles. Since the water absorbing resinparticles are irregularly pulverized, the silicon dioxide or otherwater-insoluble inorganic particles can be efficiently contained atleast either on the surface or near the surface of the water absorbingresin particles, and the physical properties of the water absorbingagent obtained are improved.

Mechanical damage refers to colliding glass, metal, etc. against thewater absorbing resin particles to give physical impact.

Mechanical damage may be given to the water absorbing resin particles byany means so long as the water absorbing resin particles receive impact.For example, a glass container, into which the water absorbing resinparticles and glass beads have been placed, may be shaken to givemechanical damage (paint shaker test, which will be further describedlater). Other methods of giving mechanical damage to the water absorbingresin particles are: a method of placing the water absorbing resinparticles in a cylindrical container together with balls and rotatingthe container (ball mill); a method of stirring in a stirrer equippedwith stirring blades; a method of passing through a paddle dryer(heater/cooler equipped with a paddle); a method of pulverizing in apulverizing device; a method of transporting pneumatically; and a methodof colliding or rubbing the particles of the water absorbing agent withone another.

In addition, to obtain a water absorbing agent with superior excellentSFC, it is especially preferable to give mechanical damage to the waterabsorbing resin particles after the particles are surface crosslinked,pulverize the particles, mix the pulverized water absorbing resinparticles with the polyvalent metal salt, then give mechanical damageagain to the water absorbing resin particles, and add the silicondioxide or other water-insoluble inorganic particles.

The water absorbing agent of the present embodiment may be manufacturedby a method of manufacturing a water absorbing agent which containswater absorbing resin particles obtained by polymerization of awater-soluble unsaturated monomer. The water absorbing resin particlesare crosslinked or coated near the surface with a surface crosslinkingagent which has at least one hydroxyl group, The water absorbing resinparticles have a mass median particle size of 200 μm to 500 μminclusive. After the water absorbing resin particles are crosslinked orcoated with the surface crosslinking agent, the polyvalent metal saltand the water-insoluble inorganic particles are mixed with the waterabsorbing resin particles. This method is called manufacturing method C.

In the manufacturing method, the water absorbing resin particles arefirst crosslinked or coated near the surface with a surface crosslinkingagent which has at least one hydroxyl group. The surface crosslinkingagent and method mentioned under the heading “Water Absorbing ResinParticles” may be employed.

The steps of manufacturing method C, except the surface crosslinkingstep, can be implemented by the corresponding steps of manufacturingmethods A, B described above.

Water Absorbent Core

The water absorbent core of the present embodiment contains the waterabsorbing agent described in the foregoing. The water absorbent core,when used in combination with an appropriate material, is suited for useas an absorbent layer in sanitary/hygienic materials, for example. Thefollowing will describe the water absorbent core.

The water absorbent core is a molded composition made of the waterabsorbing agent and other materials. The core is used in disposablediapers, sanitary napkins, incontinent pads, medical pads, and likesanitary/hygienic materials to absorb blood, body fluids, urine, etc. Anexample of the other materials used is cellulose fiber. Concreteexamples of cellulose fiber include mechanical pulp made from wood; woodpulp fibers, such as chemical pulp, semi-chemical pulp, and solublepulp; and artificial cellulose fibers, such as rayon and acetate.Preferred cellulose fiber is the wood pulp fibers. The cellulose fibermay partially contain nylon, polyester, or another synthetic fiber. Whenthe water absorbing agent of the present embodiment is used as part ofthe water absorbent core, the mass of the water absorbing agentcontained in the water absorbent core is preferably 20 mass % or more,more preferably 30 mass % or more, even more preferably 40 mass % ormore. If the mass of the water absorbing agent of the present inventioncontained in the water absorbent core is less than 20 mass %, sufficienteffects may not be accomplished.

A publicly known, suitable method for producing a water absorbent coremay be selected to produce the water absorbent core from the waterabsorbing agent of the present embodiment and the cellulose fiber. Forexample, the water absorbing agent may be sprayed onto sheets or matsmade of the cellulose fiber and sandwiching more of the agent betweenthem if necessary. Alternatively, the cellulose fiber may be uniformlyblended with the water absorbing agent. A preferred method is to dry mixthe cellulose fiber with the water absorbing agent and compress themixture. This method is highly capable of restraining the waterabsorbing agent from falling off the cellulose fiber. The compression ispreferably carried out on heating at, for example, 50° C. to 200° C.inclusive. Other preferred methods of producing an absorbent core aredescribed in the Specification of U.S. Pat. No. 5,849,405 and U.S.Published Patent Application 2003/060112.

The water absorbing agent of the present embodiment, when used in thewater absorbent core, exhibits excellent physical properties; theresultant water absorbent core is of very excellent quality in that itcan quickly absorb liquid, leaving only a little liquid on its surface.

The water absorbing agent of the present embodiment has an excellentwater absorption property and hence is applicable to waterabsorbing/retaining agents for various purposes: for example, waterabsorbing/retaining agents for absorbent articles, such as disposablediapers, sanitary napkins, incontinent pads, and medical pads;agriculture/horticulture water retaining agents, such as bog mossreplacements, soil conditioners, water retaining agents, andagricultural effect keeping agents; water retaining agents forconstruction purposes, such as dew inhibitors for interior wallmaterials and cement additives; release controlling agents; coldinsulators; disposable pocket stoves; sludge coagulating agents; foodfreshness retaining agents; ion exchange column materials; sludge/oildehydrates; desiccants; and humidity conditioners. In addition, thewater absorbing agent of the present embodiment is especially suitablefor use in disposable diapers, sanitary napkins, and likesanitary/hygienic materials for absorbing feces, urine, or blood.

Where the water absorbent core is used in sanitary/hygienic materials,such as disposable diapers, sanitary napkins, incontinent pads, andmedical pads, it is preferable if the core is placed between (a) a topsheet, permeable to liquid, provided next to the body of the user and(b) a back sheet, impermeable to liquid, provided next to the clothes ofthe user away from the body of the user. The water absorbent core may bemulti-layered (two or more layers). Further, the core may be used with apulp layer as an example.

As described in the foregoing, the water absorbing agent of the presentembodiment, in other words, is a water absorbing agent containing waterabsorbing resin particles with an internal crosslinking structureobtained by polymerization of a water-soluble unsaturated monomer. Thewater absorbing resin particles are, near the surface thereof,crosslinked with a surface crosslinking agent. The water absorbing resinparticles contain silicon dioxide at least either on the surface or nearthe surface. The silicon dioxide has, on the surface thereof, residualsilanol groups at a concentration of 1.7 SiOH/nm² or lower. The waterabsorbing resin particles contain 10 ppm to 1,900 ppm, inclusive,silicon dioxide The water absorbing agent has a mass median particlesize of 200 μm to 500 μm inclusive. The water absorbing agent contains 5mass % or less particles which have such a size that they can passthrough a sieve having a mesh opening size of 150 μm.

According to the arrangement, the water absorbing resin particles in thewater absorbing agent are crosslinked near their surface with a surfacecrosslinking agent. That reduces the amount of liquid which may seep outwhen the swollen water absorbing agent is placed under pressure. Inother words, the absorbency under 4.83 kPa pressure is increased. Inaddition, the water absorbing resin particles contain silicon dioxide atleast either on the surface or near the surface. Since the silicondioxide has excellent liquid permeability, the water absorbing agentachieves improved liquid permeability.

In addition, since the concentration of residual silanol groups in thesilicon dioxide is within the specified range, there could presumablyoccur limited hydrogen bond formation, which restrains aggregation ofthe silicon dioxide. In other words, the silicon dioxide is restrainedfrom aggregating and scattering. Therefore, unlike conventional art,even if silicon dioxide is used, dust generation due to silicon dioxideaggregation is prevented. In addition, the silicon dioxide content iswithin the specified range. Therefore, the resultant water absorbingagent has excellent physical properties and contains a reduced amount ofdust. Particles which pass through a sieve having a mesh opening size of150 μm account for 5 mass % or less of the mass of the entire waterabsorbing agent obtained. With the reduced fine particle content in thewater absorbing agent, dust is produced in a limited amount.

Thus, the resultant water absorbing agent has following features. Thesilicon dioxide and other particles in the water absorbing agent willnot fly off, unlikely to raise safety/hygiene issues, during manufactureof the water absorbing agent. Also, the physical properties of the waterabsorbent core will not likely be degraded. In addition, since the massmedian particle size of the water absorbing agent is within thespecified range, liquid permeability, etc. for the liquid to be absorbedare not disturbed; the water absorbing agent has a high liquidpermeability.

The water absorbing agent preferably contains a polyvalent metal salt atleast either on the surface or near the surface of the water absorbingresin particles in an amount of 0.001 mass % to 5 mass %, inclusive,based on the water absorbing agent.

Accordingly, the water absorbing agent contains the polyvalent metalsalt (preferably, a trivalent water-soluble polyvalent metal salt).Therefore, the water absorbing agent achieves an improved saline flowconductivity without largely reducing absorbency under 4.83 kPa pressureor fixed height absorbency. In addition, insufficient replacement ofsilicon dioxide is compensated for at least either on the surface ornear the surface of the water absorbing agent where there is no silicondioxide. In addition, synergistic effects of the silicon dioxide and thepolyvalent metal salt deliver a water absorbing agent with even betterphysical properties.

The water absorbing agent is preferably such that the water absorbingresin particles are given mechanical damage after being surfacecrosslinked.

Accordingly, the water absorbing resin particles have an irregularlypulverized shape. Therefore, the silicon dioxide can be efficientlycontained at least either on the surface or near the surface.

The water absorbing agent is preferably such that the concentration ofthe residual silanol groups on the surface of the silicon dioxide isfrom 0.7 SiOH/nm² to 1.7 SiOH/nm² inclusive.

Accordingly, the silicon dioxide is not excessively hydrophobic, butproperly so. That prevents the liquid distribution velocity of the waterabsorbing agent obtained from decreasing.

The water absorbing agent preferably has a liquid distribution velocityof 0.2 (mm/sec) to 10.0 (mm/sec) inclusive. The velocity is morepreferably from 0.5 (mm/sec) to 10.0 (mm/sec) inclusive and even morepreferably from 0.8 (mm/sec) to 10.0 (mm/sec) inclusive.

Accordingly, the liquid absorbed by the water absorbing agent diffusesefficiently. That improves the liquid suction rate per unit time of thewater absorbent core and liquid diffusibility in the water absorbentcore. Thus, water absorption capability is improved.

The water absorbing agent preferably contains 300 ppm less dust by mass.

Accordingly, the resultant water absorbing agent contains an even loweramount of dust. The silicon dioxide and other particles in the waterabsorbing agent will not fly off, unlikely to raise safety/hygieneissues. The physical properties of the water absorbing agent will notlikely be degraded.

The water absorbing agent preferably has a negative frictional electriccharge.

That prevents the silicon dioxide and the water absorbing agent fromaggregating, which in turn restrains the silicon dioxide from coming offthe water absorbing resin particles. As a result, the water absorbingagent contains a reduced amount of dust.

The water absorbent core is characterized in that it contains the waterabsorbing agent.

Accordingly, the resultant water absorbent core has high physicalproperties and contains a reduced amount of dust.

The method of manufacturing the water absorbing agent of the presentembodiment, in other words, is a method of manufacturing a waterabsorbing agent containing water absorbing resin particles with aninternal crosslinking structure obtained by polymerization of awater-soluble unsaturated monomer. The method involves crosslinking thewater absorbing resin particles near their surface with a surfacecrosslinking agent. The water absorbing resin particles have a massmedian particle size of 200 μm to 500 μm inclusive. The method furtherinvolves mixing silicon dioxide with the water absorbing resin particlesat least at a time selected from the group consisting of whilecrosslinking with the surface crosslinking agent, before thatcrosslinking, and after that crosslinking. The silicon dioxide is mixedat an amount of from 10 ppm to 1,900 ppm, inclusive, of the waterabsorbing resin particles and contains residual silanol groups at aconcentration of 1.7 SiOH/nm² or lower.

According to the method, the silicon dioxide is mixed with the waterabsorbing resin particles. The manufactured water absorbing agent hashigh physical properties and contains a reduced amount of dust.

The method of manufacturing a water absorbing agent is preferably suchthat the silicon dioxide is mixed with the water absorbing resinparticles after the water absorbing resin particles is given mechanicaldamage.

Accordingly, the silicon dioxide is mixed with the water absorbing resinparticles after the particles are pulverized into irregular shape. Thesilicon dioxide can be efficiently contained at least either on thesurface or near the surface of the water absorbing resin particles. Theresultant water absorbing agent has improved physical properties.

The method of manufacturing a water absorbing agent is preferably suchthat a polyvalent metal salt is mixed with the water absorbing resinparticles at least while crosslinking with the surface crosslinkingagent, before that crosslinking, or after that crosslinking.

Accordingly, the water absorbing agent contains the polyvalent metalsalt (preferably, a trivalent water-soluble polyvalent metal salt).Therefore, the water absorbing agent achieves an improved saline flowconductivity without largely reducing the absorbency under 4.83 kPapressure or fixed height absorbency. In addition, insufficientreplacement of silicon dioxide is compensated for at least either on thesurface or near the surface of the water absorbing agent where there isno silicon dioxide. In addition, synergistic effects of the silicondioxide and the polyvalent metal salt deliver a water absorbing agentwith even better physical properties.

The method of manufacturing a water absorbing agent preferably involvespneumatically transporting the silicon dioxide and the water absorbingresin particles while the silicon dioxide is being mixed with the waterabsorbing resin particles or after that mixing.

Accordingly, the mixture of the silicon dioxide and the water absorbingresin particles is further mixed by the pneumatic transportation. Noaggregation occurs. The silicon dioxide is more efficiently mixed withthe water absorbing resin particles. Therefore, the silicon dioxide isevenly mixed with the water absorbing resin particles. That improvesphysical properties of the water absorbing agent obtained.

The water absorbing agent of the present embodiment, in other words,contains water absorbing resin particles with an internal crosslinkingstructure obtained by polymerization of a water-soluble unsaturatedmonomer. The water absorbing resin particles are crosslinked near thesurface with an organic surface crosslinking agent and/or awater-soluble inorganic surface crosslinking agent. The water absorbingagent has a mass median particle size of 200 to 500 μm. The waterabsorbing agent contains 5 mass % or less particles which have such asize that they can pass through a sieve having a mesh opening size of150 μm. The water absorbing agent contains at least on the surfacewater-insoluble inorganic fine particles that have functional groupscapable of forming ionic bonds with functional groups on the surface ofthe water absorbing resin particles. The water-insoluble inorganic fineparticles reside on or near the surface of the water absorbing resinparticles.

The water absorbing agent is preferably such that the functional groupson the surface of the water absorbing resin particles are carboxylgroups and that the functional groups at least on the surface of thewater-insoluble inorganic fine particles are amino groups.

The water absorbing agent is preferably such that the water absorbingresin particles include particles with a porous structure.

The water absorbing agent containing a water absorbing resin and aninorganic powder that exhibits a pH from 7 to 10, inclusive, whendispersed in a liquid and a specific surface area of 50 cm²/g asmeasured by BET (see Japanese Unexamined Patent Publication (Tokukai)2000-93792) is publicly known. Since this technology does not controlthe particle size of the water absorbing resin or the quantity of fineparticles, the technology cannot solve the problems addressed by thepresent invention. In addition, the water absorbing resin hasinsufficiently liquid permeability; the absorbent core does not show asufficient liquid suction rate per unit time.

The method of manufacturing a water absorbing agent of the presentembodiment, in other words, is a method of manufacturing a waterabsorbing agent containing water absorbing resin particles with aninternal crosslinking structure obtained by polymerization of awater-soluble unsaturated monomer. The method involves mixing the waterabsorbing resin particles with water-insoluble inorganic fine particles.The water absorbing resin particles are crosslinked near the surfacewith an organic surface crosslinking agent and/or a water-solubleinorganic surface crosslinking agent. The particles have a mass medianparticle size of 200 to 500 μm. The particles include 5 mass % or lessparticles which have such a size that they can pass through a sievehaving a mesh opening size of 150 μm. The water-insoluble inorganic fineparticles have at least on the surface functional groups capable offorming ionic bonds with functional groups on the surface of the waterabsorbing resin particles. The mixing is carried out while the waterabsorbing resin particles are being surface crosslinked or in thepreceding or successive step.

The manufacturing method is preferably such that the water absorbingresin particles are crosslinked by mixing them with an organic surfacecrosslinking agent and/or a water-soluble inorganic surface crosslinkingagent and heating the mixture. The crosslinking agent contains awater-soluble inorganic salt, preferably persulfate, at an amount offrom 0.01 to 1.0 mass % based on the water absorbing resin particles.

The manufacturing method is preferably such that the water absorbingresin particles are crosslinked by mixing them with an organic surfacecrosslinking agent and/or a water-soluble inorganic surface crosslinkingagent and heating the mixture. The hydrophilic organic solvent has aboiling point of 100° C. or lower and contains no hydrophilic organicsolvent.

The manufacturing method is preferably such that the water-insolubleinorganic fine particles are added in an amount of 10 to 990 ppm.

The manufacturing method preferably further involves mixing the waterabsorbing resin particles with an at least trivalent water-solublepolyvalent metal salt in 0.001 to 5 mass % while surface crosslinking orin the preceding or successive step.

The manufacturing method preferably contains the step of pneumaticallytransporting the water absorbing agent after the mixing with thewater-insoluble inorganic fine particles.

According to the arrangement, the resultant water absorbing agent andmethod of manufacturing the water absorbing agent is such that even ifwater-insoluble inorganic fine particles are contained, fine particleswill not fly off, unlikely to raise safety/hygiene issues, in themanufacture of an absorbent core and also that the performance of theabsorbent core is not decreased.

In addition, according to the arrangement, the resultant water absorbingagent has an excellent centrifuge retention capacity (CRC) and/or salineflow conductivity (SFC) which indicate the amount of absorption by thewater absorbing agent and the liquid permeability of the water absorbingagent respectively. Therefore, the resultant water absorbing agent andmethod of manufacturing the water absorbing agent is such that theabsorbent core has an excellent liquid suction rate per unit time. Inaddition, according to the arrangement, the resultant water absorbingagent has an balance between the excellent centrifuge retention capacity(CRC) and the saline flow conductivity (SFC) which indicates liquidpermeability. Therefore, the resultant water absorbing agent and methodof manufacturing the water absorbing agent is such that the absorbentcore has an excellent liquid suction rate per unit time.

In addition, according to the arrangement, the resultant water absorbingagent and method of manufacturing the water absorbing agent is such thatthe water absorbing agent, when used in an absorbent core, impartsexcellent vertical liquid suction capability (i.e., excellent fixedheight absorbency (FHA)) to the absorbent core.

According to the arrangement, the resultant water absorbing agent has anexcellent absorbency against a pressure of 4.83 kPa (AAP). Therefore,the resultant water absorbing agent and method of manufacturing thewater absorbing agent is such that the absorbent core causes limitedliquid seeping, or “rewetting,” when it is placed under pressure.)

The water absorbing agent of the present embodiment is, in other words,is characterized in that it is a water absorbing agent containing waterabsorbing resin particles with an internal crosslinking structureobtained by polymerization of a water-soluble unsaturated monomer. Thewater absorbing agent is characterized also by the following features.The water absorbing resin particles are either crosslinked or coatednear the surface with a surface crosslinking agent which has at leastone hydroxyl group. The water absorbing resin particles contain apolyvalent metal salt and water-insoluble inorganic particles at leasteither on the surface or near the surface. The water absorbing agent hasa mass median particle size of 200 μm to 500 μm inclusive. The waterabsorbing agent contains 5 mass % or less particles which have such asize that they can pass through a sieve having a mesh opening size of150 μm.

According to the arrangement, the water absorbing resin particles in thewater absorbing agent are either crosslinked or coated near the surfacewith a surface crosslinking agent. That reduces the amount of liquidwhich may seep out when the swollen water absorbing agent is placedunder pressure. In other words, absorbency under a pressure of 4.83 kPais increased. In addition, the water absorbing resin particles contain apolyvalent metal salt and water-insoluble inorganic particles at leasteither on the surface or near the surface. The water-insoluble inorganicparticles has an excellent liquid permeability. Therefore, the waterabsorbing agent has an improved liquid permeability.

In addition, both the polyvalent metal salt and the water-insolubleinorganic particles reside at least either on the surface or near thesurface of the water absorbing resin particles. Therefore, thewater-insoluble inorganic particles are restrained from flying offpresumably by electrostatic interaction, coordinate bonds with thepolyvalent metal, and coordinate bonds mediated by water molecules inthe air, and hydrogen bonds. Therefore, unlike conventional art, even ifthe water-insoluble inorganic particles are used, dust generation due toflying water-insoluble inorganic particles is prevented. In addition,the amount of the water-insoluble inorganic particles is within thespecified range. Therefore, the resultant water absorbing agent hasexcellent physical properties and contains a reduced amount of dust.Particles which pass through a sieve having a mesh opening size of 150μm account for 5 mass % or less of the mass of the entire waterabsorbing agent obtained. With the reduced fine particle content in thewater absorbing agent, dust is produced in a limited amount.

Thus, the resultant water absorbing agent has following features. Thewater-insoluble inorganic particles and other particles in the waterabsorbing agent will not fly off, unlikely to raise safety/hygieneissues, during manufacture of the water absorbing agent. Also, thephysical properties of the water absorbent core will not likely bedegraded. In addition, since the mass median particle size of the waterabsorbing agent is within in the specified range, liquid permeability,etc. for the liquid to be absorbed are not disturbed; the waterabsorbing agent has a high liquid permeability.

The water absorbing agent preferably contains a polyvalent metal salt atleast either on the surface or near the surface of the water absorbingresin particles in an amount of 0.001 mass % to 5 mass %, inclusive,based on the water absorbing agent.

Accordingly, the resultant water absorbing agent contains the polyvalentmetal salt (preferably, a trivalent water-soluble polyvalent metalsalt). Therefore, the water absorbing agent achieves an improved salineflow conductivity without largely reducing absorbency under 4.83 kPapressure or fixed height absorbency. In addition, insufficientwater-insoluble inorganic particles are compensated for at least eitheron the surface or near the surface of the water absorbing agent wherethere are no water-insoluble inorganic particles. In addition,synergistic effects of the water-insoluble inorganic particles and thepolyvalent metal salt deliver a water absorbing agent with even betterphysical properties.

The water absorbing agent is preferably such that the water-insolubleinorganic particles are silicon dioxide.

Accordingly, interactions take place between the polyvalent metal saltresiding at least either on the surface or near the surface of the waterabsorbing resin particles and the silanol groups residing on the silicondioxide. That effectively limits the amount of dust.

The water absorbing agent is preferably such that the water absorbingresin particles are given mechanical damage after being surfacecrosslinked.

Accordingly, the water absorbing resin particles have an irregularlypulverized shape. Therefore, the water-insoluble inorganic particles canbe efficiently contained at least either on the surface or near thesurface.

The water absorbing agent preferably has a centrifuge retention capacityof 30 g/g inclusive to 50 g/g exclusive and a saline flow conductivityof 10 (10⁻⁷·cm³·s·g⁻¹) or more.

Accordingly, the resultant water absorbing agent has excellent balancebetween centrifuge retention capacity and saline flow conductivity.

The water absorbing agent preferably has an absorbency of 16 g/g to 30g/g, inclusive, under 4.83 kPa pressure.

Accordingly, the water absorbing agent discharges a reduced amount ofliquid under 4.83 kPa pressure. The resultant water absorbent coreallows only a little liquid seeping under pressure in actual use.

The water absorbing agent is preferably such that the particle sizedistribution of the agent has a logarithmic standard deviation of 0.20to 0.40 inclusive.

Accordingly, the particle size distribution is narrow. That prevents theliquid permeability of the water absorbing agent and the liquid suctionrate per unit time of the water absorbent core from decreasingnoticeably.

The water absorbing agent preferably contains 400 ppm or less dust bymass.

Accordingly, the resultant water absorbing agent contains an even loweramount of dust. The water-insoluble inorganic particles and otherparticles in the water absorbing agent will not fly off, unlikely toraise safety/hygiene issues. The physical properties of the waterabsorbing agent will not likely be degraded.

The water absorbing agent of the present embodiment, in other words, ischaracterized in that the water absorbing agent is contained.

Accordingly, the resultant water absorbent core has high physicalproperties and contains a reduced amount of dust.

The method of manufacturing a water absorbing agent of the presentembodiment, in other words, is characterized in that it is a method ofmanufacturing a water absorbing agent containing water absorbing resinparticles with an internal crosslinking structure obtained bypolymerization of a water-soluble unsaturated monomer. The method ischaracterized also in that it involves either crosslinking or coatingthe water absorbing resin particles (the mass median particle size isfrom 200 μm to 500 μm inclusive) near the surface with a surfacecrosslinking agent which has at least one hydroxyl group and mixing,after either the crosslinking or coating with the surface crosslinkingagent, the water absorbing resin particles with a polyvalent metal saltand water-insoluble inorganic particles.

According to the method, the water-insoluble inorganic particles aremixed with water absorbing resin particles and a polyvalent metal salt.The manufactured water absorbing agent has high physical properties andcontains a reduced amount of dust.

The method of manufacturing a water absorbing agent is preferably suchthat the water absorbing resin particles are mixed with a polyvalentmetal salt and water-insoluble inorganic particles after the waterabsorbing resin particles is given mechanical damage.

Accordingly, after being irregularly pulverized, the water absorbingresin particles are mixed with the polyvalent metal salt and thewater-insoluble inorganic particles. Therefore, the polyvalent metalsalt and the water-insoluble inorganic particles can be efficientlycontained at least either on the surface or near the surface of thewater absorbing resin particles. The resultant water absorbing agent hasimproved physical properties.

The method of manufacturing a water absorbing agent is preferably suchthat the water absorbing resin particles are either crosslinked orcoated with a surface crosslinking agent which has at least one hydroxylgroup before the water absorbing resin particles are mixed with apolyvalent metal salt.

Accordingly, the resultant water absorbing agent contains the polyvalentmetal salt (preferably, a trivalent water-soluble polyvalent metalsalt). Therefore, the water absorbing agent achieves an improved salineflow conductivity without largely reducing absorbency under 4.83 kPapressure or fixed height absorbency. In addition, insufficientreplacement of water-insoluble inorganic particles compensated for atleast either on the surface or near the surface of the water absorbingagent where there are no water-insoluble inorganic particles. Inaddition, synergistic effects of the water-insoluble inorganic particlesand the polyvalent metal salt deliver a water absorbing agent with evenbetter physical properties.

The method of manufacturing a water absorbing agent preferably involvespneumatically transporting the water-insoluble inorganic particles andthe water absorbing resin particles while the water-insoluble inorganicparticles are being mixed with the water absorbing resin particles orafter that mixing.

Accordingly, the mixture of the water-insoluble inorganic particles andthe water absorbing resin particles is further mixed by the pneumatictransportation. No aggregation occurs.

The water-insoluble inorganic particles are more efficiently mixed withthe water absorbing resin particles. Therefore, the water-insolubleinorganic particles are evenly mixed with the water absorbing resinparticles. That improves physical properties of the water absorbingagent obtained.

The water absorbing resin particles are preferably mixed with thewater-insoluble inorganic particles after the water absorbing resinparticles are mixed with the polyvalent metal salt. Accordingly, atleast either on the surface or near the surface of the water absorbingresin particles, the water-insoluble inorganic particles efficientlyattach to the polyvalent metal salt via electrostatic interaction,coordinate bonds with the polyvalent metal, coordinate bonds mediated bywater molecules in the air, and hydrogen bonds. The amount of dustproduced by flying water-insoluble inorganic particles is reduced.

EXAMPLES

The following will more specifically describe the present invention byway of examples. The examples are by no means limiting the presentinvention. Throughout the following, “mass parts” may be written simplyas “parts” and “liter” as “L” only for the sake of convenience. Also,“mass %” may be written as “wt %.”

The performance of the water absorbing resin particles or the waterabsorbing agent was measured by the following methods. Unless otherwisespecified, all the measurements were conducted at room temperature (20to 25° C.) and 50 RH % humidity.

In the cases of the water absorbing agent being used in an end product,such as a sanitary/hygienic material, the water absorbing agent hadalready absorbed moisture. The water absorbing agent was thereforeseparated appropriately from the end product and dried under reducedpressure and at low temperature (for example, under 1 mmHg or lower andat 60° C. for 12 hours) before measurements were made. All the waterabsorbing agents used in the examples and the comparative examplescontained 6 mass % or less water.

Centrifuge Retention Capacity (CRC)

Centrifuge retention capacity, or CRC, is absorption capacity for 0.90mass % saline under no load over 30 minutes. CRC may be referred to as“absorption capacity under no load.”

0.200 g of the water absorbing resin particles or water absorbing agentwas placed evenly in a bag (85 mm×60 mm) of non-woven fabric (“HeatronPaper” GSP-22, manufactured by Nangoku Pulp Kogyo Co., Ltd.). After heatsealing, the bag was immersed in a largely excessive amount (typicallyabout 500 mL) of 0.90 mass % saline (aqueous solution of sodiumchloride) at room temperature. After 30 minutes, the bag was taken outof the saline and centrifuged for 3 minutes in a centrifugal separator(“Centrifuge H-122,” manufactured by Kokusan Co., Ltd.) undercentrifugal force described in edana ABSORBENCY II 441.1-99 (250 G). Themass, W1 (g), of the bag was then measured. The same process was carriedout using no water absorbing resin particles or water absorbing agent,and the mass, W0 (g), of the bag was measured. The centrifuge retentioncapacity (CRC) was calculated in grams per gram from W1, WO as given bythe following equations:Centrifuge Retention Capacity (CRC) (g/g)=(W1 (g)−W0 (g))/(Mass (g) ofWater Absorbing Resin Particles or Water Absorbing Agent)−1Absorbency Against Pressure of 4.83 kPa (AAP)

Absorbency against pressure, or AAP, is absorption capacity for 0.90mass % saline under 4.83 kPa over 60 minutes. AAP may be referred to asabsorption capacity under 4.83 kPa. FIG. 1 is a cross-sectional view ofan AAP measurement apparatus 10.

In the measurement apparatus 10 shown in FIG. 1, a 400-mesh stainlesssteel net 2 (mesh opening size 38 μm) was fused to the bottom of aplastic supporter cylinder 1 that had an internal diameter of 60 mm.0.900 g of the water absorbing resin particles or water absorbing agentwas sprayed evenly on the net 2 at room temperature (from 20° C. to 25°C. inclusive) and 50 RH % humidity. A piston 4 and a weight 5 wereplaced in this order on the water absorbing resin particles or waterabsorbing agent, or the test sample 3. The piston 4 and weight 5 had anexternal diameter slightly less than 60 mm so that there occurred no gapbetween them and the supporter cylinder 1 and their up and down motionwas not disturbed. The piston 4 and weight 5 were adjusted so that theycould apply a 4.83 kPa (0.7 psi) load evenly. The mass, Wa (g), of theentire measurement apparatus 10 was measured.

A glass filter 7 measuring 90 mm in diameter (manufactured by SogoLaboratory Glass Works Co., Ltd.; pore diameter 100 to 120 μm) wasplaced inside a petri dish 6 measuring 150 mm in diameter. 0.90 mass %saline 9 (from 20° C. to 25° C. inclusive) was poured until it sit levelwith the top face of the glass filter 7. A paper filter 8 measuring 90mm in diameter (“JIS P 3801, No. 2,” Advantec Toyo Kaisha, Ltd.;thickness 0.26 mm, retainable particle size 5 μm) was placed on thefilter 7 so that the surface of the filter 8 could be all wet. Excesssolution was removed.

The whole measurement apparatus 10 was placed on the wet paper filter sothat it could absorb the solution under load. After 1 hour, the wholemeasurement apparatus 10 was lifted, and its mass Wb (g) was measured.The absorbency under 4.83 kPa (AAP) was calculated in grams per gramfrom Wa, Wb as given by the following equation:Absorbency under 4.83 kPa (AAP)=(Wb (g)−Wa (g))/(Mass of Water AbsorbingResin particles or Water Absorbing Agent (0.900 g))Saline Flow Conductivity (SFC)

Saline flow conductivity, or SFC, is a value indicating liquidpermeability of the water absorbing resin particles or water absorbingagent when they have (it has) swollen. The greater the SFC, the higherliquid permeability the water absorbing resin particles or waterabsorbing agent have/has. SFC tests were conducted in the examples asdescribed in the Specification of U.S. Pat. No. 5,849,405. FIG. 2 is aschematic illustration of an SFC measurement apparatus 20.

In the measurement apparatus 20 shown in FIG. 2, a glass tube 22 wasinserted into a tank 21. The lower end of the glass tube 22 was arrangedso that 0.69 mass % saline 23 could be maintained 5 cm above the bottomof a gel 34 in a cell 31. The 0.69 mass % saline 23 in the tank 21 wasfed to a cell 41 via a valved “L” tube 24. Under the cell 31 wasprovided a collector 38 which collected the solution that had passedthrough the cell 31. The collector 38 was placed on a balance 39. Thecell 31 had an internal diameter of 6 cm and was provided with a No. 400stainless steel net (mesh 38 μm) 32 on the bottom. The piston 36 had, onits lower part, holes 37 through which the solution could properly pass.Also, the piston 36 had a high permeability glass filter 35 attached toits bottom so that the water absorbing resin particles, water absorbingagent, or their swollen gel could not enter the holes 37. The cell 31was placed on a base. The face of the base at which it contacted thecell 31 was disposed on a stainless steel net 33 which did not disturbthe passing solution.

Artificial urine (1) used here was a mixture of 0.25 g calcium chloridedihydrate, 2.0 g potassium chloride, 0.50 g magnesium chloridehexahydrate, 2.0 g sodium sulfate, 0.85 g ammonium dihydrogen phosphate,0.15 g diammonium hydrogen phosphate, and 994.25 g pure water.

The water absorbing resin particles or water absorbing agent (0.900 g)placed evenly in the container 30 was let to swell, using themeasurement apparatus 20 shown in FIG. 2, in artificial urine (1) undera load of 2.07 kPa (0.3 psi) for 60 minutes to prepare the gel 34.Thereafter, the height of the layer of the gel 34 was recorded. Next,the 0.69 mass % saline 23 was passed through the swollen gel layer fromthe tank 21 under a load of 2.07 kPa (0.3 psi) under a constanthydrostatic pressure. The SFC test was conducted at room temperature(from 20° C. to 25° C. inclusive). The amount of liquid having passedthrough the gel layer was recorded using a computer and the balance 39as a function of time at 20 second intervals for 10 minutes. The flowrate Fs(T) at which the solution passed through the swollen gel 34(primarily between the gel's particles) was determined in units of gramsper second by dividing an increase in mass (g) by an increase in time(s). Flow rates were calculated only from the data obtained in the 10minute period starting at time Ts at which a constant hydrostaticpressure and a stable flow rate were achieved. Fs(T=0), or the firstflow rate at which the solution passed through the gel layer, wascalculated from the flow rates obtained in the 10 minute period startingat Ts. Fs(T=0) was obtained by extrapolating, for T=0, the result ofleast square approximation of Fs(T) vs. time.

$\begin{matrix}{{{Saline}\mspace{14mu}{Flow}\mspace{14mu}{Conductivity}\mspace{14mu}({SFC})} = {\left( {{{Fs}\left( {T = 0} \right)} \times L\; 0} \right)/\left( {\rho \times A \times \Delta\; P} \right)}} \\{= {\left( {{{Fs}\left( {T = 0} \right)} \times L\; 0} \right)/139506}}\end{matrix}$

where Fs(T=0) was the flow rate in grams per second; L0 was the heightof the gel layer in centimeters; ρ was the density of the NaCl solution(=1.003 g/cm³); A was the area of the top face of the gel layer in thecell 31 (=28.27 cm²); and ΔP was the hydrostatic pressure exerted on thegel layer (=4,920 dyne/cm²). The SFC values were given in units of10⁻⁷·cm³·s·g⁻¹.

Fixed Height Absorbency (FHA)

Fixed Height Absorbency, or FHA, was measured in accordance with themethod described in U.S. Published Patent Application 2005/0003191A1.The height upon measurement was set to 20 cm in the present invention.

Mass Median Particle Size D50 and Logarithmic Standard Deviation, σζ, ofParticle Size Distribution

These two parameters were measured based on the tests for the massmedian particle size, or D50, and the logarithmic standard deviation,σζ, of a particle size distribution described in InternationalApplication Published under PCT WO2004/69915.

The water absorbing resin particles or water absorbing agent was/weresieved using JIS standard sieves having various mesh opening sizes (ex.850 μm, 710 μm, 600 μm, 500 μm, 425 μm, 300 μm, 212 μm, 150 μm, and 45μm). The residual percentage R was plotted on a logarithm probabilitysheet. From the plots, the particle size which corresponded to R=50 mass% was read as the mass median particle size D50. The logarithmicstandard deviation, σζ, of a particle size distribution is given by thefollowing equation. The smaller the σζ value, the narrower the particlesize distribution.σζ=0.5×ln(X2/X1)where X1 and X2 are particle sizes for R=84.1% and R=15.9% respectively.

Classification was carried out as follows for the purpose of measuringthe mass median particle size D50 and the logarithmic standarddeviation, σζ, of the particle size distribution. 10.0 g of the waterabsorbing resin particles or water absorbing agent was/were placed ineach of JIS standard sieves (Iida Testing Sieves: 8 cm in diameter). Themesh openings of the sieves were 850 μm, 710 μm, 600 μm, 500 μm, 425 μm,300 μm, 212 μm, 150 μm, and 45 μm. The sieves were shaken at roomtemperature (from 20° C. to 25° C. inclusive) and 50 RH % humidity for 5minutes using a vibration classifier (Iida Sieve Shaker, ES-65, SER. No.0501) to classify the particles/agent.

Liquid Distribution Velocity (LDV)

Liquid distribution velocity, or LDV, was measured using a wicking indexmeasurement apparatus described in Japanese Unexamined PatentPublication 5-200068/1993 (Tokukaihei 5-200068; equivalent to EP532002). The trough sheet was prepared by SUS304, stainless steel, grade2B finish for measurement.

First, 1.00 g±0.005 g of the water absorbing resin particles or waterabsorbing agent was/were sprayed evenly from the 0 to 20 cm marks intrough grooves on a trough sheet disposed at an angle of 20°. The waterabsorbing resin particles or water absorbing agent was/were then moreevenly spread using a spatula.

The liquid to be wicked away was 0.9 mass % saline (aqueous solution ofsodium chloride) to which “Blue No. 1 for Food Testing” (available fromTokyo Chemical Industry Co., Ltd.) was added in a ratio of 0.01 g forevery 1 L of the saline for coloring.

Adjustment was made so that the liquid surface in a liquid storagevessel was 0.5 cm above the lowest point in the trough. After that,measurement of a liquid wicking time (WT) was started right when thestainless steel screen mesh (400-mesh) contacted the liquid. The liquidwicking time (WT) was the time in seconds it took for the liquid to bewicked up to the 10 cm mark. The velocity at which the liquid in theliquid storage vessel and the stainless steel screen mesh were immerseddown to 0.5 cm above the lowest point in the trough was from 1.35 to1.40 mm/s in the direction perpendicular to the liquid surface. Theliquid distribution velocity (LDV) was calculated from the followingequation:LDV (mm/s)=100 (mm)/WT (s)Ratio of Particles of Sizes which Pass Through 150-μm Mesh Openings ofSieve

The same classification process was performed as in the measurement ofthe mass median particle size D50 and the logarithmic standarddeviation, σζ, of a particle size distribution. The ratio in mass % ofthe particles of sizes that could pass through a sieve having 150-μmmesh openings was calculated from the amount of the particles that hadpassed through that sieve having the 150-μm mesh openings.

Water-Extractable Polymer Content (Water-Soluble Components)

184.3 g of 0.90 mass % saline was prepared in a lidded plastic container(capacity 250 mL). 1.00 g of the water absorbing resin particles orwater absorbing agent was added to the aqueous solution. A stirrer wasrotated for 16 hours to extract extractable polymer content in the resinby stirring the mixture. The liquid extract was filtered through a paperfilter (“JIS P 3801, No. 2,” Advantec Toyo Kaisha, Ltd.: thickness 0.26mm, retainable particle size 5 μm). 50.0 g of the obtained filtrate wasset aside for measurement as a sample solution.

First, a 0.1 N aqueous solution of NaOH was added to the 0.90 mass %saline alone, to pH 10. Then, a 0.1 N aqueous solution of HCl was addedto pH 2.7 to determine a blank titer ([bNaOH] mL, [bHCl] mL).

The same titration process was performed on the sample solution todetermine a titer ([NaOH] mL, [HCl] mL).

In the case of water absorbing resin particles or a water absorbingagent made of known amounts of an acrylic acid and its sodium salt as anexample, the extractable polymer content in the water absorbing resinparticles or water absorbing agent could be calculated according to thefollowing equation from the average molecular weight of the monomer andthe titer determined by the above-mentioned process. If the waterabsorbing resin particles or agent was/were made of unknown amounts ofan acrylic acid and its sodium salt, the average molecular weight of themonomer was calculated based on the neutralization ratio determined bythe titration.Extractable Polymer Content (mass %)=0.1×Average MolecularWeight×184.3×100×([HCl]−[bHCl])/1,000/1.0/50.0Neutralization Ratio (mol %)=(1−([NaOH]−[bNaOH])/([HCl]−[bHCl]))×100Amount of Dust (Dust Related Properties)

The increase in mass of the dust absorbed and collected by a glass fiberfilter over a predetermined period of time under the conditions detailedbelow was measured as the amount of dust in the water absorbing agent.The measurement was carried out on a Heubach Dustmeter manufactured byHeubach Engineering GmbH in Germany operating in measuring mode I. Theatmospheric conditions during the measurement were 25° C. (±2° C.)temperature, 20 to 40% relative humidity, and normal pressure. Specificprocedures were as follows.

(1) 100.00 g of a sample (water absorbing agent) was placed in arotation drum 200.

(2) The mass of the glass fiber filter 50 mm in diameter (retainableparticle size 0.5 μm (JIS P3801)) was measured with 0.00001 gramaccuracy (“Da” grams). The filter was prepared by fabricating, forexample, Advantec's glass fiber, GC-90, or any equivalent to the 50 mmdiameter.

(3) A large-scale particle separator 201 was attached to the rotationdrum 200. A filter enclosure 202 loaded with a glass fiber filter 204was also attached.

(4) Conditions were set as follows on the control section 203 of thedustmeter. Measurement was made.

Rotation Rate of Drum=30 R/min

Volume of Absorbed Air=20 L/min

Time (Measurement Period)=30 minutes

(5) After the predetermined period, the mass of the glass fiber filter204 was measured with 0.00001 gram, accuracy (“Db”).

The amount of dust was given by:Amount of Dust (ppm)=(Db−Da)/100.00×1,000,000Paint Shaker Test

In a paint shaker test (PS), a glass container 6 cm in diameter and 11cm in height was charged with 10 g of glass beads each 6 mm in diameterand 30 g of water absorbing resin particles or a water absorbing agentand loaded in a paint shaker (No. 488, Toyo Seiki Seisakusho Co., Ltd.)for shaking at 800 cycles per minute (CPM). See Japanese UnexaminedPatent Publication 9-235378/1997 (Tokukaihei 9-235378) for details ofthe device.

Tests in which the shake time was set to 30 minutes and 10 minutes weredesignated paint shaker test 1 and paint shaker test 2 respectively.

After the shaking, the glass beads were removed using a JIS standardsieve (mesh opening 2 mm), leaving behind damaged water absorbing resinparticles or a water absorbing agent.

Frictional Electric Charge

25 g of the water absorbing agent was placed in a glass screw tube andsealed. The screw tube was a screw tube No. 7, available from MaruemuCorporation, measuring 23 mm in internal diameter, 35 mm in externaldiameter, and 78 mm in height. Its cap was polypropylene, and packingwas heat resistant Highsheet.

The screw tube, in which the water absorbing agent was sealed, wasshaken continuously for 20 seconds. The shaking could be carried outusing a machine or manually. The tube was shaken 3 to 5 times persecond, and the displacement in each shake was 10 to 20 cm. The shakingneeded to be performed so that the water absorbing agent in the screwtube could be moved as greatly and quickly as possible.

After the 20-second shaking, the water absorbing agent inside the screwtube was immediately spread wide and thin on a sheet. The chargepotential of the water absorbing agent was measured using a non-contactstatic electricity meter (“FMX-002,” Simco Japan Inc.) according to theInstruction Manual, prepared by the manufacturer, which accompanied themeter. The measurement of the charge potential had to be completedwithin 15 seconds after the water absorbing agent was spread wide andthin on a sheet. The distance between the static electricity meter andthe water absorbing agent was 25 mm±1 mm during measurement asinstructed in the Manual. The charge potential display on the staticelectricity meter was immediately read over the 15-second measurementtime. If the charge potential reading indicated a negative potential,the frictional electric charge was regarded as being negative.

More specifically, if the static electricity meter showed a chargepotential in the range of +0.01 to +20.00 kv, the frictional electriccharge was regarded as being positive. If the static electricity metershowed a charge potential in the range of −20.00 to −0.01 kv, thefrictional electric charge was regarded as being negative. The chargepotential as observed in the frictional electric charge measurement ispreferably from −10.00 kv to −0.01 kv inclusive, more preferably from−5.00 kv to −0.01 kv inclusive, to keep a low transport speed stabilityindex.

The sheets used in the measurement were 12 cm×12 cm pieces cut out froma glove, “Vinytop Thick Model,” from Showa Glove Co. The outside of theglove was polyvinyl chloride resin (non-phthalate plasticizer), theinside was rayon hairs.

When the water absorbing agent was spread wide and thin on a sheet, theoutside of the glove was used as the top side of the sheet. On the topside of the sheet was spread a water absorbing agent so that the agentcould make a pile of it without flowing out of the sheet. The frictionalelectric charge was measured in this state. A “pile” of the agent refersto, for example, the water absorbing agent being spread on the sheet,forming a cone or like shape with a height of 2 to 4 cm and a bottomdiameter of 7 to 12 cm. The measurement of the frictional electriccharge was carried out in a room in which temperature was 23±2° C. andrelative humidity was 40±3%.

Proportion of SiO₂ in Dust

The proportion of SiO₂ in the dust collected in the dust quantitymeasurement was measured in mass % as the proportion of SiO₂ in thedust.

The silicon dioxide in the dust was quantified by analyzing the masspercentages of the elements sodium, aluminum, and silicon in the dustand calculating the mass ratio of the water absorbing resin and the SiO₂from results of the analysis based on the neutralization ratio andweight-average molecular weight of the water absorbing resin (in thecase of the neutralization product, or salt, is a salt of sodium).

If the neutralization salt of the water absorbing resin was a monovalentalkali salt/ammonium salt of potassium, lithium, etc., the proportion ofSiO₂ in the dust could be determined by a similar method. For example,in the case of a potassium salt, the SiO₂ proportion of the dust couldbe determined by analyzing the mass percentages of the elementspotassium, aluminum, and silicon.

The quantification analysis of the elements sodium, aluminum, silicon,etc. in the dust was carried out by ZAF using a SEM/EDS (energydispersive x-ray spectrometer).

Dust samples for the analysis were prepared by collecting appropriateamounts of dust from the glass fiber filter used in the dust quantitymeasurement and transferring them onto 5 mm×5 mm pieces of carbon tapeattached to SEM sample bases. In doing so, the dust was sprayed so thatthe dust could be distributed evenly on the carbon tape.

Conditions in the analysis are listed below.

-   Device: Scanning Electron Microscope (Scanning Microscope JSM-5410LV    from JOEL)-   Acceleration Voltage: 20 kv-   Magnification: 20×-   Measurement Area: about 900 μm×1,200 μm-   Measurement carried out with at least 50 volume % or more of the    entire measurement area being covered with dust.-   Si Peak SiK 1.739 KeV-   Na Peak: NaK 1.041 KeV-   Al Peak AlK 1.486 KeV

Note that if these peaks overlapped the peak of another element (forexample, NaK and ZnLa), the value of the peak of the overlapping, otherelement (ZnKa in the case of Zn) was subtracted for correction.

From the mass percentage of the element sodium (hereinafter, may bewritten simply as “Na %”), the mass percentage of the element aluminum(hereinafter, may be written simply as “Al %”), the mass percentage ofthe element silicon (hereinafter, may be written simply as “Si %”), theneutralization ratio N of the water absorbing resin in mol % (detailedlater), and a polymer-unit-weight-average molecular weight Mw, theproportion of SiO₂ in the dust was given in mass % by the followingequations:Polymer-unit-weight-average Molecular WeightMw=72.06×(1−N/100)+94.05×N/100Polymer Content P=(Na %/23)/(N/100)×MwSiO₂ Content S=(Si %/28.08)×60.08Aluminum Sulfate Content A=(Al %/26.98)×630.4/2Proportion of SiO₂ in Dust in mass %=S/(P+S+A)×100

In the equations, “N” was the neutralization ratio of the waterabsorbing resin and could be measured by the same method as theaforementioned method for water-extractable polymer content measurement.

The SiO₂ proportion of the dust is preferably measured by theaforementioned method. When there are unknown components or many otherelements, elemental analysis or another conventional publicly knownmethod may be used for the measurement.

Flyability of Dust

100 g of a water absorbing agent was placed in a stainless steel funnel(steel designation X5 CrNiMo 17-12-3 specified in ISO/TR 15,510;internal diameter 10 mm; height 145 mm; tilt angle 20°). That funnelcontaining the water absorbing agent was dropped from the height of 30cm into a cylindrical beaker 8 cm in diameter and 12 cm in height. Itwas visually inspected how easily dust was blown up when the funnel hitthe beaker. In the measurement, a blackboard was placed close to thecylindrical beaker to make it easier to inspect dust being produced.

The dust's flyability was divided into the following 5 levels.

-   Flyability 1: Hardly blown up.-   Flyability 2: Blown up a little.-   Flyability 3: Blown up.-   Flyability 4: Blown up rather lot.-   Flyability 5: Blown up much.    Quantification Method for Silicon Dioxide in Water Absorbing Agent

The amount of silicon dioxide in the water absorbing agent can bemeasured by elemental analysis or a like publicly known method. Anymeasuring method may be used. The following is a mere example.

(1) 0.500 g of a water absorbing agent is placed in a polypropylenebeaker (capacity 250 mL). 0.5 g of sodium carbonate (anhydrous) ofreagent grade was added.

(2) 100 mL of deionized water (grade 3, ISO 3696) at 80° C. was added tothe polypropylene beaker using a 100-mL plastic graduated cylinder. Thecontent of the beaker was stirred for 2 hours while being maintained at80° C. to dissolve solid silica.

(3) The content was filtered through a pleated quantification paperfilter (No. 5C, 185 mm, Toyo Roshi Kaisha, Ltd.) in a plastic funnel.The filtrate was received in a 100-mL plastic graduated flask.

(4) When there remained little liquid on the paper filter in the plasticfunnel (≅1 hour later), 3 mL of 6N hydrochloric acid was added using aplastic Komagome pipette so that gel could contract as much as possible.

(5) 3 mL of 6N hydrochloric acid was added to the obtained filtrate.Subsequently, 4 mL of a 5 mass % solution of ammonium molybdate wasadded twice. An appropriate amount of the deionized water was added toincrease the volume of the content. The graduated flask was sealed andshaken well.

(6) The optical absorbency (ABS) of the colored solution obtained in (5)was measured using a spectrophotometer (IU-1100, Hitachi, Ltd.;wavelength 410 nm, cell thickness 10 mm) within 5 to 20 seconds afterthe coloring occurred. The same process was performed on a blank sampleusing deionized water (grade 3, ISO 3696).

(7) The optical absorbency value of the blank was subtracted from theobtained optical absorbency of the colored solution. The resultant valuewas designated the optical absorbency of the test sample. The amount ofsilicon dioxide fine particles in the test sample was determined in mass% based on a calibration curve (described below).

Preparation of Calibration Curve

Standard samples were prepared by adding 0 mass parts, 0.03 mass parts,0.06 mass parts, 0.1 mass parts, 0.2 mass parts, 0.3 mass parts, 0.5mass parts, 1.0 mass part of silicon dioxide fine particles (forexample, Aerosil (Registered Trademark) 200 from Nippon Aerosil Co.,Ltd.) respectively to 100 mass parts of a water absorbing resincontaining no silicon dioxide (for example, water absorbing resin (1-30)which will be detailed later).

The optical absorbencies of these standard samples of which the silicondioxide fine particle content in mass % is known were determined by theaforementioned process. The calibration curve was drawn from theresultant optical absorbency values.

EXAMPLE 1

436.4 g of an acrylic acid, 4,617.9 g of a 37 mass % aqueous solution ofsodium acrylate, 381.0 g of pure water, and 11.40 g of polyethyleneglycol diacrylate (molecular weight 523) were dissolved in a reactorwhich was a lidded double-arm stainless steel kneader (internal volume10 liters) equipped with two sigma-type blades and a jacket, to preparea reaction solution. Next, the reaction solution was deaerated in anitrogen gas atmosphere for 20 minutes. Subsequently, 38.76 g of a 10mass % aqueous solution of sodium persulfate and 24.22 g of a 0.1 mass %aqueous solution of L-ascorbic acid were added to the reaction solutionwhile stirring, about seconds after which polymerization started. Thepolymerization was let to proceed at 25 to 95° C., while crushing theproduced gel. The water-containing gel-like crosslinked polymer wasremoved 30 minutes into the polymerization. The resultingwater-containing gel-like crosslinked polymer had been comminuted to asize of about 5 mm or less.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 180° C. for 50 minutes.The dried substance was pulverized in a roll mill and subjected to aclassification using JIS standard sieves having mesh opening sizes of710 μm and 175 μm. The result was water absorbing resin particles (1)which had an irregularly pulverized shape. Particles (1) had a massmedian particle size D50 of 341 μm. The logarithmic standard deviation,σζ, of the particle size distribution of particles (1) was 0.33. Waterabsorbing resin particles (1) had a centrifuge retention capacity (CRC)of 35.4 g/g and contained a 7.3 mass % water-extractable polymercontent.

100 mass parts of water absorbing resin particles (1) obtained wasevenly mixed with a surface crosslinking agent that was a mixed solutionof 0.384 mass parts of 1,4-butanediol, 0.632 mass parts of propyleneglycol, 3.39 mass parts of pure water, and 0.1 mass parts of sodiumpersulfate. The mixture was then heat treated at 212° C. Differentmixture samples were prepared with different heating times: 30 minutes,35 minutes, 40 minutes, and 45 minutes. Thereafter, the resultingparticles were disintegrated until they could pass through a JISstandard sieve having a mesh opening size of 710 μm. Next, thedisintegrated particles were subjected to paint shaker test 1, toprepare surface-crosslinked water absorbing resin particle samples: oneof the samples was heated for 30 minutes to prepare water absorbingresin particles (1-30), another one for 35 minutes to prepare waterabsorbing resin particles (1-35), another one for 40 minutes to preparewater absorbing resin particles (1-40), and another one for 45 minutesto prepare water absorbing resin particles (1-45).

A solution was then added to each 100 mass part sample of thesurface-crosslinked water absorbing resin particles obtained. Thesolution was a mixture of 0.40 mass parts of a 27.5 mass % aqueoussolution of aluminum sulfate (equivalent to an 8 mass % aqueous solutionof aluminum oxide), 0.134 mass parts of a 60 mass % aqueous solution ofsodium lactate, and 0.002 mass parts of propylene glycol. After theaddition, the samples were dried in a windless environment at 60° C. for1 hour. Following the drying, the samples were disintegrated until theycould pass through a JIS standard sieve having a mesh opening size of710 μm. Next, the disintegrated samples were subjected to paint shakertest 2, to prepare water absorbing resin particle samples: waterabsorbing resin particles (1-30A) was prepared from water absorbingresin particles (1-30), water absorbing resin particles (1-35A) fromwater absorbing resin particles (1-35), water absorbing resin particles(1-40A) from water absorbing resin particles (1-40), and water absorbingresin particles (1-45A) from water absorbing resin particles (1-45).

0.040 mass parts of HDK (Registered Trademark), H2050EP, from Wacker wasadded to 100 mass parts of water absorbing resin particles (1-30A),(1-35A), (1-40A), and (1-45A) to prepare water absorbing agents (1-1),(1-2), (1-3), and (1-4) respectively.

Likewise,

0.070 mass parts of HDK (Registered Trademark), H2050EP, from Wacker wasadded to 100 mass parts of water absorbing resin particles (1-30A),(1-35A), (1-40A), and (1-45A) to prepare water absorbing agents (1-5),(1-6), (1-7), and (1-8) respectively.

COMPARATIVE EXAMPLE 1

The following experiments were conducted in reference to referentialexample 1, example 1, and example 2 of Japanese Unexamined PatentPublication (Tokukai) 2000-93792.

1.70 mass parts of trimethylolpropane triacrylate was dissolved in 5,500mass parts of an aqueous solution of sodium acrylate (monomerconcentration 38%) which had a neutralization ratio of 75 mol %, toprepare a reaction solution. Next, the reaction solution was deaeratedin a nitrogen gas atmosphere for 30 minutes. The reaction solution wasthen fed to a reactor which was a lidded double-arm stainless steelkneader (internal volume 10 liters) equipped with two sigma-type bladesand a jacket. Nitrogen gas was substituted for the system while keepingthe reaction solution at 30° C. Subsequently, 2.46 mass parts of sodiumpersulfate and 0.10 mass parts of L-ascorbic acid were added to thereaction solution while stirring, about 1 minute after whichpolymerization started. The polymerization was let to proceed at 30° C.to 80° C. inclusive. The water-containing gel-like polymer was removed60 minutes into the polymerization. The resulting water-containinggel-like polymer had been comminuted to a size of about 5 mm.

The comminuted water-containing gel-like polymer was spread on a 50-meshmetal net and dried in hot wind at 150° C. for 90 minutes. The driedsubstance was pulverized in a vibration mill and subjected to aclassification using a 20-mesh metal net. The result was irregularlypulverized water absorbing resin precursor (A) with a mean particle sizeof 330 μm.

100 mass parts of water absorbing resin precursor (A) obtained was mixedwith a surface crosslinking agent that was a mixed solution of 1 masspart of propylene glycol, 0.05 mass parts of ethylene glycol diglycidylether, 3 mass parts of water, and 1 mass part of isopropyl alcohol. Themixture was then heat treated at 210° C. for 40 minutes, to obtaincomparative water absorbing resin particles (1-1). The absorptioncapacity and absorption capacity against pressure of comparative waterabsorbing resin particles (1-1) were measured by the method described inJapanese Unexamined Patent Publication (Tokukai) 2000-93792; the CRC was31 g/g, and the AAP was 33 g/g.

0.05 mass parts of hydrophobic silicon dioxide

(RA200HS manufactured by Nippon Aerosil Co., Ltd.), as inorganic powder,was mixed with 100 mass parts of comparative water absorbing resinparticles (1-1), to obtain comparative water absorbing agent (1-1). Thehydrophobic silicon dioxide was a 4% cationic dispersion liquid with apH of 7.5 or higher and a specific surface area of 145±15 m²/g asmeasured by BET.

100 mass parts of water absorbing resin precursor (A) obtained was mixedwith a surface crosslinking agent that was a mixed solution of 0.1 massparts of ethylene glycol diglycidyl ether, 4.5 mass parts of water, and1.5 mass parts of isopropyl alcohol. The mixture was heat treated at200° C. for 35 minutes, to obtain comparative water absorbing resinparticles (1-2). The absorption capacity and absorption capacity againstpressure of comparative water absorbing resin particles (1-2) weremeasured by the method described in Japanese Unexamined PatentPublication (Tokukai) 2000-93792; the CRC was 32 g/g, and the AAP was 31g/g.

1 mass part of water was added to 100 mass parts of comparative waterabsorbing resin particles (1-2). Thereafter, 0.1 mass parts of theinorganic powder (RA200HS manufactured by Nippon Aerosil Co., Ltd.) wasmixed with the result similarly to the foregoing case, to obtaincomparative water absorbing agent (1-2).

Table 1 shows measurements of physical properties of water absorbingresin particles (1), (1-30), (1-35), (1-40), (1-45), (1-30A), (1-35A),(1-40A), and (1-45A), water absorbing agent (1-1), (1-2), (1-3), (1-4),(1-5), (1-6), (1-7), and (1-8), comparative water absorbing resinparticles (1-1) and (1-2), and comparative water absorbing agents (1-1),(1-2).

EXAMPLE 2

436.4 g of an acrylic acid, 4,617.9 g of a 37 mass % aqueous solution ofsodium acrylate, 381.0 g of pure water, and 11.40 g of polyethyleneglycol diacrylate (molecular weight 523) were dissolved in a reactorwhich was a lidded double-arm stainless steel kneader (internal volume10 liters) equipped with two sigma-type blades and a jacket, to preparea reaction solution. Next, the reaction solution was deaerated in anitrogen gas atmosphere for 20 minutes. Subsequently, 29.07 g of a 10mass % aqueous solution of sodium persulfate and 24.22 g of a 0.1 mass %aqueous solution of L-ascorbic acid were added to the reaction solutionwhile stirring, about 1 minute after which polymerization started. Thepolymerization was let to proceed at 25 to 95° C., while crushing theproduced gel. The water-containing gel-like crosslinked polymer wasremoved 30 minutes into the polymerization. The resultingwater-containing gel-like crosslinked polymer had been comminuted to asize of about 5 mm or less.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 180° C. for 50 minutes.The dried substance was pulverized in a roll mill and subjected to aclassification using JIS standard sieves having mesh opening sizes of710 μm and 175 μm. The result was water absorbing resin particles (2)which had an irregularly pulverized shape. Particles (2) had a massmedian particle size D50 of 341 μm. The logarithmic standard deviation,σζ, of the particle size distribution of particles (2) was 0.33. Waterabsorbing resin particles (2) had a centrifuge retention capacity (CRC)of 33.5 g/g and contained a 9.0 mass % water-extractable polymercontent.

100 mass parts of water absorbing resin particles (2) obtained wasevenly mixed with a surface crosslinking agent that was a mixed solutionof 0.384 mass parts of 1,4-butanediol, 0.632 mass parts of propyleneglycol, 3.39 mass parts of pure water, and 0.1 mass parts of sodiumpersulfate. The mixture was then heat treated at 212° C. for 50 minutes.The particles were disintegrated until they could pass through a JISstandard sieve having a mesh opening size of 710 μm. Next, thedisintegrated particles were subjected to paint shaker test 1, to obtainsurface-crosslinked water absorbing resin particles (2-50).

Different amounts of HDK (Registered Trademark), H2050EP, from Wackerwere added to 100 mass part samples of water absorbing resin particles(2-50): 0.010 mass parts of the HDK was added to one of the samples toprepare water absorbing agent (2-1), 0.020 mass parts to prepare waterabsorbing agent (2-2), 0.040 mass parts to prepare water absorbing agent(2-3), 0.070 mass parts to prepare water absorbing agent (2-4), and0.100 mass parts to prepare water absorbing agent (2-5).

COMPARATIVE EXAMPLE 2

0.300 mass parts of HDK (Registered Trademark), H2050EP, from Wacker wasadded to 100 mass parts of water absorbing resin particles (2-50)obtained in example 2 to prepare comparative water absorbing agent(2-1). 0.3000 mass parts of Aerosil (Registered Trademark) 200 fromNippon Aerosil Co., Ltd. was added to another 100 mass parts of waterabsorbing resin particles (2-50) to prepare comparative water absorbingagent (2-2).

Table 2 shows measurements of some physical properties of waterabsorbing resin particles (2) and (2-50), water absorbing agents (2-1),(2-2), (2-3), (2-4), and (2-5), and comparative water absorbing agents(2-1) and (2-2).

EXAMPLE 3

Solution (A1) and solution (B1) were mixed in a polypropylene container(internal diameter 80 mm; internal volume 1 liter) covered with styrenefoam (heat insulation material). The mixing was performed quickly in anopen system by adding solution (B1) to solution (A1) while stirring witha magnetic stirrer. Solution (A1) was a mixture of 220.81 g of anacrylic acid, 1.154 g of polyethylene glycol diacrylate (molecularweight 523), and 1.35 g of a 1.0 mass % aqueous solution of pentasodiumdiethylenetriaminepentaacetate. Solution (B1) was a mixture of 184.49 gof a 48.5 mass % aqueous solution of sodium hydroxide and 179.94 g ofion-exchanged water of which the temperature was adjusted to 50° C. Theresult of the mixing was a monomer aqueous solution, of which thetemperature had risen to about 100° C. due to heat of neutralization anddissolution.

12.26 g of a 3 mass % aqueous solution of sodium persulfate was added tothe obtained monomer aqueous solution. After stirring several seconds,the solution was poured into a tray-type stainless steel container in anopen system. For the pouring, the container had been heated on a hotplate (Neo Hotplate H1-1000, manufactured by As One Corporation) so thatthe surface temperature reached 100° C. The container had a bottom(250×250 mm) the inside of which was coated with Teflon (RegisteredTrademark). Its top was 640×640 mm and height was 50 mm. Thecross-section of its mid-section was trapezoidal. Its top was open.

Soon after the monomer aqueous solution was poured into the tray,polymerization started. The polymerization proceeded producing watervapor, with the solution foamed/expanded in every direction. Thereafter,the content shrank to a size a little larger than the tray bottom. Theexpansion and shrink finished in about 1 minute. After being left in thecontainer for 4 minutes, the water-containing polymer was removed.

The obtained water-containing polymer was crushed using a meat chopperwith a dice size of 9.5 mm (Royal Meat Chopper VR400K manufactured byIidzuka Industries Co., Ltd.) to obtain a comminuted water-containingpolymer.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 180° C. for 50 minutes.The dried substance was then pulverized in a roll mill and subjected toa classification using JIS standard sieves having mesh opening sizes of710 μm and 175 μm. The result was water absorbing resin particles (3)which had an irregularly pulverized shape. Particles (3) had a massmedian particle size D50 of 329 μm. The logarithmic standard deviation,σζ, of the particle size distribution of particles (3) was 0.31. Waterabsorbing resin particles (3) had a centrifuge retention capacity (CRC)of 33.0 g/g and contained a 9.0 mass % water-extractable polymercontent.

100 mass parts of water absorbing resin particles (3) obtained was mixedwith a surface crosslinking agent that was a mixed solution of 0.36 massparts of 1,4-butanediol, 0.6 mass parts of propylene glycol, and 3.2mass parts of pure water. The mixture was then mixed using a Loedigemixer and heat treated at 200° C. for 50 minutes. The particles weredisintegrated until they could pass through a JIS standard sieve havinga mesh opening size of 710 μm. Next, the disintegrated particles weresubjected to paint shaker test 1, to obtain surface-crosslinked waterabsorbing resin particles (3-50).

A solution was added to 100 mass parts of surface-crosslinked waterabsorbing resin particles (3-50) obtained. The solution was a mixture of0.40 mass parts of a 27.5 mass % aqueous solution of aluminum sulfate(equivalent to an 8 mass % aqueous solution of aluminum oxide), 0.134mass parts of a 60 mass % aqueous solution of sodium lactate, and 0.002mass parts of propylene glycol. After the addition, the particles weredried in a windless environment at 60° C. for 1 hour. Following thedrying, the particles were disintegrated until they could pass through aJIS standard sieve having a mesh opening size of 710 μm. Next, thedisintegrated particles were subjected to paint shaker test 2, toobtained water absorbing resin particles (3-50A).

Different amounts of HDK (Registered Trademark), H2050EP, from Wackerwere added to 100 mass part samples of water absorbing resin particles(3-50A): 0.020 mass parts of the HDK was added to one of the samples toprepare water absorbing agent (3-1), 0.040 mass parts to prepare waterabsorbing agent (3-2), 0.060 mass parts to prepare water absorbing agent(3-3), 0.080 mass parts to prepare water absorbing agent (3-4), 0.0990mass parts to prepare water absorbing agent (3-5), 0.1250 mass parts toprepare water absorbing agent (3-6), and 0.1500 mass parts to preparewater absorbing agent (3-7).

EXAMPLE 4

0.100 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to a 100 mass part sample of water absorbingresin particles (3-50A) prepared in example 3 to obtain water absorbingagent (4-1). 0.1250 mass parts of the Aerosil was added to another 100mass part sample of particles (3-50A) to obtain water absorbing agent(4-2).

Table 3 shows measurements physical properties of water absorbing resinparticles (3), (3-50), and (3-50A), water absorbing agents (3-1), (3-2),(3-3), (3-4), (3-5), (3-6), and (3-7), and water absorbing agents (4-1)and (4-2).

EXAMPLE 5

Solution (A) and solution (B) were mixed in a polypropylene container(internal diameter 80 mm; internal volume 1 liter) covered with styrenefoam (heat insulation material). The mixing was performed quickly in anopen system by adding solution (B) to solution (A) while stirring with amagnetic stirrer. Solution (A) was a mixture of 221.92 g of an acrylicacid, 1.53 g of polyethylene glycol diacrylate (molecular weight 523),and 1.35 g of a 1.0 mass % aqueous solution of pentasodiumdiethylenetriaminepentaacetate. Solution (B) was a mixture of 180.33 gof a 48.5 mass % aqueous solution of sodium hydroxide and 182.55 g ofion-exchanged water of which the temperature was adjusted to 50° C. Theresult of the mixing was a monomer aqueous solution, of which thetemperature had risen to about 100° C. due to heat of neutralization anddissolution.

12.32 g of a 3 mass % aqueous solution of sodium persulfate was added tothe obtained monomer aqueous solution. After stirring several seconds,the solution was poured into a tray-type stainless steel container in anopen system. For the pouring, the container had been heated on a hotplate (Neo Hotplate H1-1000, manufactured by As One Corporation) so thatthe surface temperature reached 100° C. The container had a bottom(250×250 mm) the inside of which was coated with Teflon (RegisteredTrademark). Its top was 640×640 mm and height was 50 mm. Thecross-section of its mid-section was trapezoidal. Its top was open.

Soon after the monomer aqueous solution was poured into the tray,polymerization started. The polymerization proceeded producing watervapor, with the solution foamed/expanded in every direction. Thereafter,the content shrank to a size a little larger than the tray bottom. Theexpansion and shrink finished in about 1 minute. After being left in thecontainer for 4 minutes, the water-containing polymer was removed.

The obtained water-containing polymer was crushed using a meat chopperwith a dice size of 9.5 mm (Royal Meat Chopper VR400K manufactured byIidzuka. Industries Co., Ltd.) to obtain a comminuted water-containingpolymer.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 180° C. for 50 minutes.The dried substance was then pulverized in a roll mill and subjected toa classification using JIS standard sieves having mesh opening sizes of710 μm and 175 μm. The result was water absorbing resin particles (5)which had an irregularly pulverized shape. Particles (5) had a massmedian particle size D50 of 342 μm. The logarithmic standard deviation,cg, of the particle size distribution of particles (5) was 0.34. Waterabsorbing resin particles (5) had a centrifuge retention capacity (CRC)of 31.0 g/g and contained a 8.0 mass % water-extractable polymercontent.

100 mass parts of water absorbing resin particles (5) obtained wasevenly mixed with a surface crosslinking agent that was a mixed solutionof 0.31 mass parts of 1,4-butanediol, 0.49 mass parts of propyleneglycol, and 2.4 mass parts of pure water. The mixture was then heattreated at 195° C. for 50 minutes. The particles were disintegrateduntil they could pass through a JIS standard sieve having a mesh openingsize of 710 μm. Next, the disintegrated particles were subjected topaint shaker test 1, obtain surface-crosslinked water absorbing resinparticles (5-50).

A solution was added to 100 mass parts of surface-crosslinked waterabsorbing resin particles (5-50) obtained. The solution was a mixture of0.50 mass parts of a 27.5 mass % aqueous solution of aluminum sulfate(equivalent to an 8 mass % aqueous solution of aluminum oxide), 0.16mass parts of a 60 mass % aqueous solution of sodium lactate, and 0.0025mass parts of propylene glycol.

After the addition, the particles were dried in a windless environmentat 60° C. for 1 hour. Following the drying, the particles weredisintegrated until they could pass through a JIS standard sieve havinga mesh opening size of 710 μm. Next, the disintegrated particles weresubjected to paint shaker test 2, obtain water absorbing resin particles(5-50A).

Different amounts of HDK (Registered Trademark), H2050EP, from Wackerwere added to 100 mass part samples of water absorbing resin particles(5-50A): 0.040 mass parts of the HDK was added to one of the samples toprepare water absorbing agent (5-1), 0.080 mass parts to prepare waterabsorbing agent (5-2), and 0.1250 mass parts to prepare water absorbingagent (5-3).

Table 4 shows measurements of physical properties of water absorbingresin particles (5), (5-50), (5-50A) and water absorbing agents (5-1),(5-2), and (5-3).

TABLE 1 Amount Inorganic Amount of Particles Added CRC AAP SFC FHA D50*2 Dust Added ppm g/g g/g *1 g/g μm mass % ppm WARPs (1) 35.4 9.5 0 3411.8 WARPs (1-30) 31.0 26.7 17 345 1.6 WARPs (1-35) 29.7 26.4 37 343 1.7WARPs (1-40) 27.8 25.5 49 341 1.9 WARPs (1-45) 27.0 24.9 44 339 2 WARPs(1-30A) 31.0 26.1 45 25.4 346 1.3 233 WARPs (1-35A) 30.1 25.7 79 24.8345 1.5 235 WARPs (1-40A) 28.1 24.9 115 23.7 343 1.7 236 WARPs (1-45A)27.7 24.6 132 22.8 341 1.9 238 WAA (1-1) HDK H2050EP 400 31.0 25.0 5525.3 346 1.3 235 WAA (1-2) HDK H2050EP 400 30.1 24.5 92 24.1 345 1.5 236WAA (1-3) HDK H2050EP 400 28.1 24.1 117 23.1 343 1.7 238 WAA (1-4) HDKH2050EP 400 27.7 23.6 126 22.5 341 1.9 238 WAA (1-5) HDK H2050EP 70031.0 24.6 58 24.1 346 1.3 236 WAA (1-6) HDK H2050EP 700 30.1 24.0 11123.0 345 1.5 238 WAA (1-7) HDK H2050EP 700 28.1 23.5 129 22.8 343 1.7238 WAA (1-8) HDK H2050EP 700 27.7 22.8 142 22.1 341 1.9 238 C-WARPs(1-1) 29.7 23.3 13 343 6.8 358 C-WARPs (1-2) 30.8 23.8 7 345 5.9 352C-WAA (1-1) RA200HS 500 29.7 22.9 29 21.7 343 6.8 360 C-WAA (1-2)RA200HS 1000 30.8 22.1 20 21.3 345 5.9 360 *1 Units: 10⁻⁷ · cm³ · s ·g⁻¹ *2 Ratio of Particles that Passed Through Sieve Having Mesh OpeningSize of 150 μm WARPs: Water Absorbing Resin Particles WAA: WaterAbsorbing Agent C-WARPs: Comparative Water Absorbing Resin ParticlesC-WAA: Comparative Water Absorbing Agent

As shown in Table 1, water absorbing resin particles (1-30) to (1-45),obtained by adding a surface crosslinking agent to samples of waterabsorbing resin particles (1), surface-crosslinking the mixtures, andsubjecting the results to paint shaker test 1 to create mechanicaldamage, exhibited relatively low CRCs, but improved AAPs and SFCs whencompared to water absorbing resin particles (1).

A comparison of water absorbing resin particles (1-30) to (1-45) clearlyshows that the physical properties of the obtained water absorbing resinparticles can be adjusted by changing heating time.

Water absorbing resin particles (1-30A) to (1-45A), obtained by treatingwater absorbing resin particles (1-30) to (1-45) with an aqueoussolution of aluminum sulfate and subjecting the results to paint shakertest 2 to create mechanical damage, exhibited slightly low AAPs, butgreatly improved SFCs.

Water absorbing agents (1-1) to (1-4), obtained by adding silicondioxide, which is hydrophobic, to water absorbing resin particles(1-30A) to (1-45A), practically retained good CRCs and AAPs and alsoexhibited very high SFCs. In other words, very useful water absorbingagents were obtained with high CRC, AAP, and SFC values.

Comparative water absorbing resin particles (1-1) and (1-2), althoughmanufactured using hydrophobic silicon dioxide, contained more than 5mass % particles that could pass through a sieve having a mesh openingsize of 150 μm because the particles did not undergo classificationusing a JIS standard sieves. The same holds true with the cases ofcomparative water absorbing agents (1-1) and (1-2). These comparativewater absorbing resin particles and comparative water absorbing agentshad low liquid permeability because they contain high ratios ofparticles that could pass through a sieve having a mesh opening size of150 μm. Accordingly, their SFCs were very low. Their AAPs and FHAs werealso low. There occurred much dust.

TABLE 2 Amount Inorganic Amount of Particles Added CRC AAP SFC FHA D50*2 Dust Added ppm g/g g/g *1 g/g μm mass % ppm WAPRs (2) 33.5 9.1 0 3411.3 WAPRs (2-50) 28.6 25.2 38 23.5 338 1.5 208 WAA (2-1) HDK H2050EP 10028.6 24.4 55 24.3 339 1.5 208 WAA (2-2) HDK H2050EP 200 28.6 24.4 5824.3 340 1.5 216 WAA (2-3) HDK H2050EP 400 28.6 23.9 70 24.1 340 1.5 219WAA (2-4) HDK H2050EP 700 28.6 23.3 71 23.6 342 1.6 219 WAA (2-5) HDKH2050EP 1000 28.5 23.1 81 19.2 344 1.6 245 C-WAA (2-1) HDK H2050EP 300028.5 22.8 111 19.0 344 1.8 340 C-WAA (2-2) Aerosil 200 3000 28.5 22.0103 19.0 344 1.8 393 *1 Units: 10⁻⁷ · cm³ · s · g⁻¹ *2 Ratio ofParticles that Passed Through Sieve Having Mesh Opening Size of 150 μmWARPs: Water Absorbing Resin Particles WAA: Water Absorbing AgentC-WARPs: Comparative Water Absorbing Resin Particles C-WAA: ComparativeWater Absorbing Agent

All the water absorbing resin particles and water absorbing agents inTable 2 had far smaller amounts of dust than comparative water absorbingagents (2-1) and (2-2). Although the same amount of inorganic particleswas added to comparative water absorbing agents (2-1) and (2-2),comparative water absorbing agent (2-1) produced less dust and showedbetter SFC and AAP.

Water absorbing agent (2-1) to (2-4) had especially high FHAs.

TABLE 3 Inorganic Amount Amount of Particles Added CRC AAP SFC FHA D50*2 Dust Added ppm g/g g/g *1 g/g μm mass % ppm WAPRs (3) 33.0 9.3 0 3291.6 WAPRs (3-50) 27.5 24.1 31 328 1.5 WAPRs (3-50A) 27.3 23.9 65 23.4330 1.7 210 WAA (3-1) HDK H2050EP 200 27.3 23.4 78 23.0 330 1.7 210 WAA(3-2) HDK H2050EP 400 27.5 23.1 78 22.9 331 1.7 220 WAA (3-3) HDKH2050EP 600 27.5 23.0 79 22.7 330 1.7 221 WAA (3-4) HDK H2050EP 800 27.422.9 85 22.4 331 1.8 225 WAA (3-5) HDK H2050EP 990 27.3 22.9 88 22.5 3311.8 230 WAA (3-6) HDK H2050EP 1250 27.5 22.8 101 21.7 332 1.8 252 WAA(3-7) HDK H2050EP 1500 27.6 22.4 94 20.8 335 1.9 260 WAA (4-1) Aerosil200 1000 27.3 22.1 87 21.1 332 1.8 308 WAA (4-2) Aerosil 200 1250 27.522.0 92 20.3 334 1.8 310 *1 Units: 10⁻⁷ · cm³ · s · g⁻¹ *2 Ratio ofParticles that Passed Through Sieve Having Mesh Opening Size of 150 μmWARPs: Water Absorbing Resin Particles WAA: Water Absorbing AgentC-WARPs: Comparative Water Absorbing Resin Particles C-WAA: ComparativeWater Absorbing Agent

All the water absorbing resin particles and water absorbing agents inTable 3 (especially, water absorbing agents (3-1) to (3-7)) had a verysmall amount of dust. Specifically, although the same amount ofinorganic particles was added to water absorbing agent (3-6) andcomparative water absorbing agent (4-2), water absorbing agent (3-6)produced less dust.

Water absorbing agents (3-1) to (3-5) exhibited particularly good FHAs.

TABLE 4 Inorganic Amount Amount of Particles Added CRC AAP SFC FHA D50*2 Dust Added ppm g/g g/g *1 g/g μm mass % ppm WAPRs (5) 31.0 9.5 0 3281.4 WAPRs (5-50) 26.0 93.7 52 329 1.4 WAPRs (5-50A) 25.7 23.2 96 22.3329 1.3 220 WAA (5-1) HDK H2050EP 400 25.8 22.7 110 21.7 331 1.3 218 WAA(5-2) HDK H2050EP 800 25.5 22.4 141 21.5 330 1.4 224 WAA (5-3) HDKH2050EP 1250 25.8 22.3 156 21.0 330 1.4 255 *1 Units: 10⁻⁷ · cm³ · s ·g⁻¹ *2 Ratio of Particles that Passed Through Sieve Having Mesh OpeningSize of 150 μm WARPs: Water Absorbing Resin Particles WAA: WaterAbsorbing Agent C-WARPs: Comparative Water Absorbing Resin ParticlesC-WAA: Comparative Water Absorbing Agent

As shown in Table 4, water absorbing agents (5-1) to (5-3) exhibitedexcellent liquid permeabilities (SFCs) and produced very little dust.

EXAMPLE 6

The same process was carried out as in example 1, except that 0.050 massparts of HDK (Registered Trademark), H20, from Wacker, in place of 0.040mass parts of HDK (Registered Trademark), H2050EP, from Wacker, wasadded to 100 mass parts of water absorbing resin particles (1-30A),(1-35A), (1-40A), and (1-45A), to obtain water absorbing agents (6-1),(6-2), (6-3), and (6-4) respectively.

Table 5 shows the kind and amount of silicon dioxide added, as well asmeasurements of the CRC, AAP, SFC, FHA, D50, and the ratio of particlesthat passed through a sieve having a mesh opening size of 150 μm, forwater absorbing agents (6-1), (6-2), (6-3) and (6-4).

TABLE 5 Amount of Silicon Amount Silicon Dioxide of Dioxide Added CRCAAP SFC FHA D50 *2 Dust Added ppm g/g g/g *1 g/g μm mass % ppm WAA (6-1)HDK H20 500 30.9 25.1 60 25.3 345 1.3 211 WAA (6-2) HDK H20 500 29.724.5 115 24.3 345 1.5 212 WAA (6-3) HDK H20 500 28.0 24.3 132 23.4 3441.7 215 WAA (6-4) HDK H20 500 27.6 23.7 156 22.9 343 1.9 220 *1 Units:10⁻⁷ · cm³ · s · g⁻¹ *2 Ratio of Particles that Passed Through SieveHaving Mesh Opening Size of 150 μm WARPs: Water Absorbing ResinParticles WAA: Water Absorbing Agent C-WARPs: Comparative WaterAbsorbing Resin Particles C-WAA: Comparative Water Absorbing Agent

As shown in Table 5, water absorbing agents (6-1) to (6-4) exhibitedexcellent liquid permeabilities (SFCs) and high AAPs and FHAs, andproduced very little dust.

EXAMPLE 7

436.4 g of an acrylic acid, 4,617.9 g of a 37 mass % aqueous solution ofsodium acrylate, 381.0 g of pure water, and 7.6 g of polyethylene glycoldiacrylate (molecular weight 523) were dissolved in a reactor which wasa lidded double-arm stainless steel kneader (internal volume 10 liters)equipped with two sigma-type blades and a jacket, to prepare a reactionsolution. Next, the reaction solution was deaerated in a nitrogen gasatmosphere for 20 minutes. Subsequently, 29.07 g of a 10 mass % aqueoussolution of sodium persulfate and 24.22 g of a 0.1 mass % aqueoussolution of L-ascorbic acid were added to the reaction solution whilestirring, about minute after which polymerization started. Thepolymerization was let to proceed at 25 to 95° C., while crushing theproduced gel. The water-containing gel-like crosslinked polymer wasremoved 30 minutes into the polymerization. The resultingwater-containing gel-like crosslinked polymer had been comminuted to asize of about 5 mm or less.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 180° C. for 50 minutes.The dried substance was pulverized in a roll mill and subjected to aclassification using JIS standard sieves having mesh opening sizes of710 μm and 175 μm. The result was water absorbing resin particles (7)which had an irregularly pulverized shape. Particles (7) had a massmedian particle size D50 of 471 μm. The logarithmic standard deviation,σζ, of the particle size distribution of particles (7) was 0.37. Waterabsorbing resin particles (7) had a centrifuge retention capacity (CRC)of 38.0 g/g and contained a 11.0 mass % water-extractable polymercontent.

100 mass parts of water absorbing resin particles (7) obtained wasevenly mixed with a surface crosslinking agent that was a mixed solutionof 0.3 mass parts of 1,4-butanediol, 0.5 mass parts of propylene glycol,and 2.7 mass parts of pure water. The mixture was then heat treated at212° C. for 40 minutes. The particles were disintegrated until theycould pass through a JIS standard sieve having a mesh opening size of850 μm. Next, the disintegrated particles were subjected to paint shakertest 1, to obtain surface-crosslinked water absorbing resin particles(7-40).

A solution was then added to each 100 mass part sample of the waterabsorbing resin particles (7-40) obtained. The solution was a mixture of0.40 mass parts of a 27.5 mass % aqueous solution of aluminum sulfate(equivalent to an 8 mass % aqueous solution of aluminum oxide), 0.134mass parts of a 60 mass % aqueous solution of sodium lactate, and 0.002mass parts of propylene glycol. After the addition, the samples weredried in a windless environment at 60° C. for 1 hour. Following thedrying, the samples were disintegrated until they could pass through aJIS standard sieve having a mesh opening size of 710 μm. Next, thedisintegrated samples were subjected to paint shaker test 2, to obtainwater absorbing resin particles (7-40A).

Different amounts of HDK (Registered Trademark), H20, from Wacker wereadded to 100 mass part samples of water absorbing resin particles(7-40A) obtained: 0.020 mass parts of the HDK was added to one of thesamples to prepare water absorbing agent (7-1), 0.040 mass parts toprepare water absorbing agent (7-2), 0.070 mass parts to prepare waterabsorbing agent (7-3), and 0.100 mass parts to prepare water absorbingagent (7-4).

Table 6 shows the kind and amount of silicon dioxide added, as well asmeasurements of the CRC, AAP, SFC, FHA, LDV, D50, and ratio of particlesthat passed through a sieve having a mesh opening size of 150 μm, forwater absorbing resin particles (7), (7-40), and (7-40A) and waterabsorbing agents (7-1), (7-2), (7-3), and (7-4). The amounts of dustwere also measured, the results of which are shown in Table 6.

EXAMPLE 8

0.200 mass parts of HDK (Registered Trademark), H20, from Wacker wasadded to 100 mass parts water absorbing resin particles (7-40A) preparedin example 7 to obtain water absorbing agent (8-1).

0.200 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to 100 mass parts of water absorbing resinparticles (7-40A) prepared in example 7 to obtain water absorbing agent(8-2).

0.200 mass parts of Aerosil (Registered Trademark), R-972, frommanufactured by Nippon Aerosil Co., Ltd. was added to 100 mass parts ofwater absorbing resin particles (7-40A) prepared in example 7, to obtainwater absorbing agent (8-3).

Table 6 shows the kind and amount of silicon dioxide added, as well asmeasurements of the CRC, AAP, SFC, FHA, LDV, D50, and the ratio ofparticles that passed through a sieve having a mesh opening size of 150μm, for water absorbing agents (8-1), (8-2) and (8-3). The amounts ofdust were also measured, the results of which are shown in Table 6.

TABLE 6 Amount of Silicon Amount Silicon Dioxide of Dioxide Added CRCAAP SFC FHA LDV D50 *2 Dust Added ppm g/g g/g *1 g/g mm/sec μm mass %ppm WAPRs (7) 38.0 8.6 0 471 1.5 WAPRs (7-40) 29.8 25.4 28 471 1.6 WAPRs(7-40A) 29.4 24.9 47 22.3 0.72 472 1.5 179 WAA (7-1) HDK H20 200 29.724.3 54 22.6 0.79 470 1.6 180 WAA (7-2) HDK H20 400 29.5 23.2 56 21.90.82 472 1.6 207 WAA (7-3) HDK H20 700 29.7 23.2 65 21.5 0.73 473 1.6189 WAA (7-4) HDK H20 1000 29.2 22.7 75 21.3 0.84 471 1.6 245 WAA (8-1)HDK H20 2000 29.0 21.1 83 19.6 0.84 471 1.9 301 WAA (8-2) Aerosil 2002000 28.5 20.8 76 19.4 1.80 472 1.8 350 WAA (8-3) Aerosil R-972 200029.0 21.0 86 18.9 0.29 472 1.9 302 *1 Units: 10⁻⁷ · cm³ · s · g⁻¹ *2Ratio of Particles that Passed Through Sieve Having Mesh Opening Size of150 μm WARPs: Water Absorbing Resin Particles WAA: Water Absorbing AgentC-WARPs: Comparative Water Absorbing Resin Particles C-WAA: ComparativeWater Absorbing Agent

The measurements for water absorbing agents (7-1) to (7-4) and (8-1) to(8-3) in Table 6 demonstrate that water absorbing agents were obtainedwhich produced very small amounts of dust and showed high SFCs. Theamount of dust was less than or equal to 400 ppm for any of waterabsorbing agents (7-1) to (7-4) and (8-1) to (8-3). Water absorbingagents were manufactured which were unlikely to produce dust.

Water absorbing agents (7-1) to (7-4) exhibited particularly good AAPsand FHAs. Water absorbing agents (7-1) to (7-4), (8-1), and (8-2)exhibited excellent LDVs. The amount of dust was particularly low forwater absorbing agents (7-1) to (7-3) and a little low for waterabsorbing agents (7-4). The amount of dust was also low for waterabsorbing agents (8-1) and (8-3). Although the same amount of silicondioxide was added to water absorbing agents (8-1) to (8-3), waterabsorbing agent (8-2) produced more dust and exhibited a lower AAP andSFC than water absorbing agents (8-1) and (8-3).

EXAMPLE 9

Solution (A) and solution (B) were mixed in a polypropylene container(internal diameter 80 mm; internal volume 1 liter) covered with styrenefoam (heat insulation material). The mixing was performed quickly in anopen system by adding solution (B) to solution (A) while stirring with amagnetic stirrer. Solution (A) was a mixture of 221.92 g of an acrylicacid, 1.53 g of polyethylene glycol diacrylate (molecular weight 523),and 1.35 g of a 1.0 mass % aqueous solution of pentasodiumdiethylenetriaminepentaacetate. Solution (B) was a mixture of 180.33 gof a 48.5 mass % aqueous solution of sodium hydroxide and 182.55 g ofion-exchanged water of which the temperature was adjusted to 50° C. Theresult of the mixing was a monomer aqueous solution, of which thetemperature had risen to about 100° C. due to heat of neutralization anddissolution.

12.32 g of a 3 mass % aqueous solution of sodium persulfate was added tothe obtained monomer aqueous solution. After stirring several seconds,the solution was poured into a tray-type stainless steel container in anopen system. For the pouring, the container had been heated on a hotplate (Neo Hotplate H1-1000, manufactured by As One Corporation) so thatthe surface temperature reached 100° C. The container had a bottom(250×250 mm) the inside of which was coated with Teflon (RegisteredTrademark). Its top was 640×640 mm and height was 50 mm. Thecross-section of its mid-section was trapezoidal. Its top was open.

Soon after the monomer aqueous solution was poured into the tray,polymerization started. The polymerization proceeded producing watervapor, with the solution foamed/expanded in every direction. Thereafter,the content shrank to a size a little larger than the tray bottom. Theexpansion and shrink finished in about 1 minute. After being left in thecontainer for 4 minutes, the water-containing polymer was removed.

The obtained water-containing polymer was crushed using a meat chopperwith a dice size of 9.5 mm (Royal Meat Chopper VR400K manufactured byIidzuka Industries Co., Ltd.) to obtain a comminuted water-containingpolymer.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 180° C. for 50 minutes.The dried substance was then pulverized in a roll mill and subjected toa classification using JIS standard sieves having mesh opening sizes of710 μm and 175 μm. The result was water absorbing resin particles (9)which had an irregularly pulverized shape. Particles (9) had a massmedian particle size D50 of 342 μm. The logarithmic standard deviation,σζ, of the particle size distribution of particles (9) was 0.34. Waterabsorbing resin particles (9) had a centrifuge retention capacity (CRC)of 31.0 g/g and contained a 8.0 mass % water-extractable polymercontent.

100 mass parts of water absorbing resin particles (9) obtained wasevenly mixed with a surface crosslinking agent that was a mixed solutionof 0.31 mass parts of 1,4-butanediol, 0.49 mass parts of propyleneglycol, and 2.4 mass parts of pure water. The mixture was then heattreated at 195° C. for 50 minutes. The particles were disintegrateduntil they could pass through a JIS standard sieve having a mesh openingsize of 710 μm. Next, the disintegrated particles were subjected topaint shaker test 1, to obtain surface-crosslinked water absorbing resinparticles (9-50).

A solution was added to 100 mass parts of surface-crosslinked waterabsorbing resin particles (9-50) obtained. The solution was a mixture of0.40 mass parts of a 27.5 mass % aqueous solution of aluminum sulfate(equivalent to an 8 mass % aqueous solution of aluminum oxide), 0.16mass parts of a 60 mass % aqueous solution of sodium lactate, and 0.0025mass parts of propylene glycol. After the addition, the particles weredried in a windless environment at 60° C. for 1 hour. Following thedrying, the particles were disintegrated until they could pass through aJIS standard sieve having a mesh opening size of 710 μm. Next, thedisintegrated particles were subjected to paint shaker test 2, obtainwater absorbing resin particles (9-50A).

Different amounts of HDK (Registered Trademark), H20, from Wacker wereadded to 100 mass part samples of water absorbing resin particles(9-50A) obtained: 0.020 mass parts of the HDK was added to one of thesamples to prepare water absorbing agent (9-1), 0.040 mass parts toprepare water absorbing agent (9-2), 0.070 mass parts to prepare waterabsorbing agent (9-3), and 0.100 mass parts to prepare water absorbingagent (9-4).

Different amounts of Aerosil (Registered Trademark), R-972, frommanufactured by Nippon Aerosil Co., Ltd. were added to 100 mass partsamples of water absorbing resin particles (9-50A) obtained: 0.040 massparts of the Aerosil was added to one of the samples to prepare waterabsorbing agent (9-5), 0.070 mass parts to prepare water absorbing agent(9-6), and 0.100 mass parts to prepare water absorbing agent (9-7).

Different amounts of HDK (Registered Trademark), H15, from Wacker, wereadded to 100 mass part samples of water absorbing resin particles(9-50A) obtained: 0.020 mass parts of the HDK was added to one of thesamples to prepare water absorbing agent (9-8), 0.040 mass parts toprepare water absorbing agent (9-9), 0.070 mass parts to prepare waterabsorbing agent (9-10), and 0.100 mass parts to prepare water absorbingagent (9-11).

Table 7 shows the kind and amount of silicon dioxide added, as well asmeasurements of the CRC, AAP, SFC, FHA, LDV, D50, and the ratio ofparticles that passed through a sieve having a mesh opening size of 150μm, for water absorbing resin particles (9), (9-50) and (9-50A) andwater absorbing agents (9-1) to (9-11). The amounts of dust were alsomeasured, the results of which are shown in Table 7.

TABLE 7 Amount of Silicon Amount Silicon Dioxide of Dioxide Added CRCAAP SFC FHA LDV D50 *2 Dust Added ppm g/g g/g *1 g/g mm/sec μm mass %ppm WAPRs (9) 31.0 9.5 0 328 1.4 WAPRs (9-50) 26.0 23.7 52 329 1.4 WAPRs(9-50A) 25.7 23.2 96 22.3 1.01 329 1.3 233 WAA (9-1) HDK H20 200 25.523.0 112 22.3 1.15 331 1.3 235 WAA (9-2) HDK H20 400 25.5 23.0 131 21.91.07 330 1.3 248 WAA (9-3) HDK H20 700 26.0 23.0 139 21.5 1.36 332 1.3230 WAA (9-4) HDK H20 1000 25.8 22.9 149 21.3 1.04 331 1.4 245 WAA (9-5)Aerosil R-972 400 25.7 22.9 129 21.6 0.73 332 1.3 237 WAA (9-6) AerosilR-972 700 25.8 22.7 166 21.3 0.52 333 1.3 238 WAA (9-7) Aerosil R-9721000 25.9 21.6 173 21.1 0.42 332 1.4 249 WAA (9-8) HDK H15 200 25.4 23.6126 22.3 1.14 333 1.4 230 WAA (9-9) HDK H15 400 25.6 22.7 135 22.0 1.11333 1.3 231 WAA (9-10) HDK H15 700 25.7 22.4 154 21.5 1.36 334 1.4 231WAA (9-11) HDK H15 1000 25.9 22.2 163 21.4 1.15 332 1.4 244 *1 Units:10⁻⁷ · cm³ · s · g⁻¹ *2 Ratio of Particles that Passed Through SieveHaving Mesh Opening Size of 150 μm WARPs: Water Absorbing ResinParticles WAA: Water Absorbing Agent C-WARPs: Comparative WaterAbsorbing Resin Particles C-WAA: Comparative Water Absorbing Agent

A comparison of water absorbing agents (9-1) to (9-4) in Table 7 clearlyshows that the SFC improves with increasing amount of silicon dioxideadded. The same holds true with the cases of water absorbing agents(9-5) to (9-7) and water absorbing agents (9-8) to (9-11).

As shown in Table 7, the amount of dust is less than or equal to 300 ppmfor any of water absorbing resin particles (9-50A) and water absorbingagents (9-1) to (9-11). Water absorbing agents were manufactured whichwere unlikely to produce dust.

Water absorbing agents (9-1) to (9-4) and (9-8) to (9-11) exhibitedexcellent LDVs.

EXAMPLE 10

425.2 g of an acrylic acid, 4,499.5 g of a 37 mass % aqueous solution ofsodium acrylate, 538.5 g of pure water, and 6.17 g of polyethyleneglycol diacrylate (molecular weight 523) were dissolved in a reactorwhich was a lidded double-arm stainless steel kneader (internal volume10 liters) equipped with two sigma-type blades and a jacket, to preparea reaction solution. Next, the reaction solution was deaerated in anitrogen gas atmosphere for 20 minutes. Subsequently, 28.3 g of a 10mass % aqueous solution of sodium persulfate and 23.6 g of a 0.1 mass %aqueous solution of L-ascorbic acid were added to the reaction solutionwhile stirring, about 25 seconds after which polymerization started. Thepolymerization was let to proceed at 25° C. to 95° C. inclusive, whilecrushing the produced gel. The water-containing gel-like crosslinkedpolymer was removed 30 minutes into the polymerization. The resultingwater-containing gel-like crosslinked polymer had been comminuted to asize of about 5 mm or less.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 170° C. for 65 minutes.The dried substance was pulverized in a roll mill and subjected to aclassification using a JIS standard sieve having a mesh opening size of850 μm. The result was water absorbing resin particles (10) which had anirregularly pulverized shape. Particles (10) had a mass median particlesize D50 of 458 μm. The logarithmic standard deviation, σζ, of theparticle size distribution of particles (10) was 0.40. Water absorbingresin particles (10) had a centrifuge retention capacity (CRC) of 42 g/gand contained a 13 mass % water-extractable polymer content.

100 mass parts of water absorbing resin particles (10) obtained wasevenly mixed with a surface crosslinking agent that was a mixed solutionof 0.35 mass parts of 1,4-butanediol, 0.55 mass parts of propyleneglycol, and 3.0 mass parts of pure water. The mixture was then heattreated at 212° C. for 40 minutes. Thereafter, the resulting particleswere disintegrated until they could pass through a JIS standard sievehaving a mesh opening size of 850 μm. Next, the disintegrated particleswere subjected to paint shaker test 1, to obtain surface-crosslinked or-coated water absorbing resin particles (10).

A solution was then added to 100 mass parts of surface-crosslinked or-coated water absorbing resin particles (10). The solution was a mixtureof 0.9 mass parts of a 27.5 mass % aqueous solution of aluminum sulfate(equivalent to an 8 mass % aqueous solution of aluminum oxide), 0.134mass parts of a 60 mass % aqueous solution of sodium lactate, and 0.025mass parts of propylene glycol. After the addition, the particles weredried in a windless environment at 60° C. for 1 hour. Following thedrying, the particles were disintegrated until they could pass through aJIS standard sieve having a mesh opening size of 850 μm. The waterabsorbing resin particles obtained from water absorbing resin particles(10) were designated (10-A).

0.20 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts of waterabsorbing resin particles (10-A) obtained above, to prepare waterabsorbing agent (10). Table 8 shows the kind and amount of thewater-insoluble inorganic particles added, as well as measurements ofthe CRC, AAP, SFC, D50, and ratio of particles that passed through asieve having a mesh opening size of 150 μm, and amount of dust, forwater absorbing resin particles (10-A) and water absorbing agent (10).

EXAMPLE 11

0.30 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts of waterabsorbing resin particles (10-A) prepared in example 10 to obtain waterabsorbing agent (11). Table 8 shows the kind and amount of thewater-insoluble inorganic particles added, as well as measurements ofthe CRC, AAP, SFC, D50, ratio of particles that passed through a sievehaving a mesh opening size of 150 μm, and amount of dust, for waterabsorbing agent (11) obtained.

EXAMPLE 12

The same method was used as in example 10 until a comminutedwater-containing gel-like crosslinked polymer was obtained.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 170° C. for 65 minutes.The dried substance was pulverized in a roll mill and subjected to aclassification using a JIS standard sieve having a mesh opening size of850 μm. The result was water absorbing resin particles (12) which had anirregularly pulverized shape. Particles (12) had a mass median particlesize D50 of 330 μm. The logarithmic standard deviation, σζ, of theparticle size distribution of particles (12) was 0.35. Water absorbingresin particles (12) had a centrifuge retention capacity (CRC) of 42 g/gand contained a 13 mass % water-extractable polymer content.

100 mass parts of water absorbing resin particles (12) obtained wasevenly mixed with a surface crosslinking agent that was a mixed solutionof 0.35 mass parts of 1,4-butanediol, 0.55 mass parts of propyleneglycol, and 3.0 mass parts of pure water. The mixture was then heattreated at 212° C. for 40 minutes. The resultant particles weredisintegrated until they could pass through a JIS standard sieve havinga mesh opening size of 850 μm. Next, the disintegrated particles weresubjected to paint shaker test 1, to obtain surface-crosslinked or-coated water absorbing resin particles (12).

A solution was then added to 100 mass parts of surface-crosslinked or-coated water absorbing resin particles (12). The solution was a mixtureof 0.9 mass parts of a 27.5 mass % aqueous solution of aluminum sulfate(equivalent to an 8 mass % aqueous solution of aluminum oxide), 0.134mass parts of a 60 mass % aqueous solution of sodium lactate, and 0.025mass parts of propylene glycol. After the addition, the particles weredried in a windless environment at 60° C. for 1 hour. Following thedrying, the particles were disintegrated until they could pass through aJIS standard sieve having a mesh opening size of 850 μm. The waterabsorbing resin particles obtained from water absorbing resin particles(12) were designated (12-A).

0.20 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts of waterabsorbing resin particles (12-A) obtained above, to prepare waterabsorbing agent (3C). Table 8 shows the kind and amount of thewater-insoluble inorganic particles added, as well as measurements ofthe CRC, AAP, SFC, D50, ratio of particles that passed through a sievehaving a mesh opening size of 150 μm, and amount of dust, for waterabsorbing agent (12).

EXAMPLE 13

A substance was added to 100 mass parts of surface-crosslinked or-coated water absorbing resin particles (10) prepared in example 10. Thesubstance was a mixture of 0.9 mass parts of a 27.5 mass % aqueoussolution of aluminum sulfate (equivalent to an 8 mass % aqueous solutionof aluminum oxide), 0.134 mass parts of a 60 mass % aqueous solution ofsodium lactate, 0.025 mass parts of propylene glycol, and as a plantcomponent, 0.5 mass parts of a 15 mass % aqueous solution of an extractfrom leaves of a theaceous plant which contained polyphenol and caffeine(“FS-80MO” available from Shiraimatsu Pharmaceutical Co., Ltd., locatedat 37-1, Ukawa, Mizuguchi-cho, Kouga-gun, Shiga)). After the addition,the particles were dried in a windless environment at 60° C. for 1 hour.Following the drying, the particles were disintegrated until they couldpass through a JIS standard sieve having a mesh opening size of 850 μm,to prepare water absorbing resin particles (13-A).

0.20 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts of waterabsorbing resin particles (13-A) obtained above, to prepare waterabsorbing agent (13). Table 8 shows the kind and amount of thewater-insoluble inorganic particles added, as well as measurements ofthe CRC, AAP, SFC, D50, ratio of particles that passed through a sievehaving a mesh opening size of 150 μm, and amount of dust, for waterabsorbing agent (13).

EXAMPLE 14

A substance was added to 100 mass parts of surface-crosslinked or-coated water absorbing resin particles (12) prepared in example 12. Thesubstance was a mixture of 0.9 mass parts of a 27.5 mass % aqueoussolution of aluminum sulfate (equivalent to an 8 mass % aqueous solutionof aluminum oxide), 0.134 mass parts of a 60 mass % aqueous solution ofsodium lactate, 0.025 mass parts of propylene glycol, and as a plantcomponent, 0.5 mass parts of a 15 mass % aqueous solution of an extractfrom leaves of a theaceous plant which contained polyphenol and caffeine(“FS-80MO” available from Shiraimatsu Pharmaceutical Co., Ltd., locatedat 37-1, Ukawa, Mizuguchi-cho, Kouga-gun, Shiga)). After the addition,the particles were dried in a windless environment at 60° C. for 1 hour.Following the drying, the particles were disintegrated until they couldpass through a JIS standard sieve having a mesh opening size of 850 μm,to prepare water absorbing resin particles (14-A).

0.20 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts of waterabsorbing resin particles (14-A) obtained above, to prepare waterabsorbing agent (14). Table 8 shows the kind and amount of thewater-insoluble inorganic particles added, as well as measurements ofthe CRC, AAP, SFC, D50, ratio of particles that passed through a sievehaving a mesh opening size of 150 μm, and amount of dust, for waterabsorbing agent (14).

COMPARATIVE EXAMPLE 3

0.20 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts of waterabsorbing resin particles (10) prepared in example 10 to obtaincomparative water absorbing agent (3). Table 8 shows the kind and amountof the water-insoluble inorganic particles added, as well asmeasurements of the CRC, AAP, SFC, D50, ratio of particles that passedthrough a sieve having a mesh opening size of 150 μm, and amount ofdust, for comparative water absorbing agent (3) obtained.

COMPARATIVE EXAMPLE 4

0.30 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts of waterabsorbing resin particles (10) obtained in example 10 to obtaincomparative water absorbing agent (4). Table 8 shows the kind and amountof the water-insoluble inorganic particles added, as well asmeasurements of the CRC, AAP, SFC, D50, ratio of particles that passedthrough a sieve having a mesh opening size of 150 μm, and amount ofdust, for comparative water absorbing agent (4) obtained.

COMPARATIVE EXAMPLE 5

The same method was used as in example 10 until a comminutedwater-containing gel-like crosslinked polymer was obtained.

The comminuted water-containing gel-like crosslinked polymer was spreadon a 50-mesh metal net and dried in hot wind at 170° C. for 65 minutes.The dried substance was pulverized in a roll mill and subjected to aclassification using a JIS standard sieve having a mesh opening size of850 μm. The result was comparative water absorbing resin particles (5)which had an irregularly pulverized shape. Comparative particles (5) hada mass median particle size D50 of 440 μm. The logarithmic standarddeviation, σζ, of the particle size distribution of comparativeparticles (5) was 0.50. Comparative water absorbing resin particles (5)had a centrifuge retention capacity (CRC) of 42 g/g and contained a 13mass % water-extractable polymer content.

100 mass parts of obtained comparative water absorbing resin particles(5) was evenly mixed with a surface crosslinking agent that was a mixedsolution of 0.35 mass parts of 1,4-butanediol, 0.55 mass parts ofpropylene glycol, and 3.0 mass parts of pure water. The mixture was thenheat treated at 212° C. for 40 minutes.

The resultant particles were disintegrated until they could pass througha JIS standard sieve having a mesh opening size of 850 μm. Next, thedisintegrated particles were subjected to paint shaker test 1, to obtainsurface-crosslinked or -coated comparative water absorbing resinparticles (5).

A solution was then added to 100 mass parts of surface-crosslinked or-coated comparative water absorbing resin particles (5). The solutionwas a mixture of 0.9 mass parts of a 27.5 mass % aqueous solution ofaluminum sulfate (equivalent to an 8 mass % aqueous solution of aluminumoxide), 0.134 mass parts of a 60 mass % aqueous solution of sodiumlactate, and 0.025 mass parts of propylene glycol. After the addition,the particles were dried in a windless environment at 60° C. for 1 hour.Following the drying, the particles were disintegrated until they couldpass through a JIS standard sieve having a mesh opening size of 850 μm.The comparative water absorbing resin particles obtained fromcomparative water absorbing resin particles (5) was designated (5-A).

0.20 mass parts of Aerosil (Registered Trademark) 200 from NipponAerosil Co., Ltd. was added to and mixed with 100 mass parts ofcomparative water absorbing resin particles (5-A) obtained above, toprepare comparative water absorbing agent (5). Table 8 shows the kindand amount of the water-insoluble inorganic particles added, as well asmeasurements of the CRC, AAP, SFC, D50, ratio of particles that passedthrough a sieve having a mesh opening size of 150 μm, and amount ofdust, for comparative water absorbing resin particles (5-A) andcomparative water absorbing agent (5) obtained.

TABLE 8 Inorganic Amount Amount of *3 Particles Added CRC AAP SFC D50 *2Dust mass % Added ppm g/g g/g *1 μm mass % ppm WAPRs (10-A) 0.9 — — 33.922.2 5 458 2.5 215 WAA (10) 0.9 Aerosil 200 2000 34.1 19.5 15 469 3.0310 WAA (11) 0.9 Aerosil 200 3000 34.2 19.1 17 460 3.1 330 WAA (12) 0.9Aerosil 200 2000 34.2 19.5 12 341 3.2 322 WAA (13) 0.9 Aerosil 200 200034.2 19.3 14 463 2.8 301 WAA (14) 0.9 Aerosil 200 2000 34.0 19.2 12 3423.1 302 C-WAA (3) — Aerosil 200 2000 34.2 19.0 5 463 3.0 483 C-WAA (4) —Aerosil 200 3000 34.0 18.6 6 467 3.4 524 C-WAPRs (5-A) 0.9 — — 33.8 20.50 443 6.3 232 C-WAA (5) 0.9 Aerosil 200 2000 33.9 18.1 6 442 6.5 420 *1Units: 10⁻⁷ · cm³ · s · g⁻¹ *2 Ratio of Particles that Passed ThroughSieve Having Mesh Opening Size of 150 μm *3 Amount of 27.5 mass %Aqueous Solution of Aluminum Sulfate Added WARPs: Water Absorbing ResinParticles WAA: Water Absorbing Agent C-WARPs: Comparative WaterAbsorbing Resin Particles C-WAA: Comparative Water Absorbing Agent

A comparison of water absorbing agent (10) to comparative waterabsorbing agent (3) and water absorbing agent (11) to comparative waterabsorbing agent (4) in Table 8 shows that the use of both a polyvalentmetal salt and water-insoluble inorganic particles improved the SFC andreduced the amount of dust while limiting degradation of the AAP.Furthermore, a comparison of water absorbing agent (10) to comparativewater absorbing agent (5) show that the SFC decreased at high ratios ofparticles that passed through a sieve having a mesh opening size of 150μm.

In addition, the water absorbing resin particles and water absorbingagents to which aluminum sulfate was added produced little dust andexhibited excellent SFCs and AAPs.

Table 9 shows measurements of the SiO₂ content in dust and the dust'sflyability for the water absorbing agents and comparative waterabsorbing agents mentioned above.

TABLE 9 Amount Ratio of *3 Inorganic Amount of SiO₂ Flyability massParticles Added Dust in Dust of % Added ppm ppm mass % Dust WAA (1-7)0.40 HDK 700 219 8.0 2 H2050EP WAA (9-6) 0.40 Aerosil 700 238 8.1 2R-972 WAA (9-3) 0.40 HDK H20 700 230 5.2 1 WAA (9-4) 0.40 HDK H20 1000245 9.1 2 WAA (10) 0.90 Aerosil 2000 310 41.5 4 200 WAA (11) 0.90Aerosil 3000 330 43.1 4 200 WAA (12) 0.90 Aerosil 2000 322 41.9 4 200WAA (4-1) 0.40 Aerosil 1000 308 32.0 3 200 WAA (4-2) 0.4 Aerosil 1250310 34.1 3 200 WAA (8-2) 0.4 Aerosil 2000 350 45.2 4 200 C-WAA — Aerosil3000 393 66.6 5 (2-2) 200 C-WAA (3) — Aerosil 2000 483 68.2 5 200 C-WAA(4) — Aerosil 3000 524 74.6 5 200 *2 Ratio of Particles that PassedThrough Sieve Having Mesh Opening Size of 150 μm *3 Amount of 27.5 mass% Aqueous Solution of Aluminum Sulfate Added WARPs: Water AbsorbingResin Particles WAA: Water Absorbing Agent C-WARPs: Comparative WaterAbsorbing Resin Particles C-WAA: Comparative Water Absorbing Agent

As could be appreciated from Table 9, dust is unlikely to rise when theSiO₂ content in the dust is low. It would also be appreciated that thelower the SiO₂ content in the dust, the less likely dust rises.

As such, the SiO₂ content of the dust is preferably 50 mass % or less,more preferably 30 mass % or less, even more preferably 15 mass % orless, still more preferably 10 mass % or less, most preferably 7 mass %or less.

The present invention is not limited to the description of theembodiments above, but may be altered by a skilled person within thescope of the claims. An embodiment based on a proper combination oftechnical means disclosed in different embodiments is encompassed in thetechnical scope of the present invention.

Industrial Applicability

The water absorbing agent and water absorbent core in accordance withthe present invention and the water absorbing agent obtained by themanufacturing method for a water absorbing agent in accordance with thepresent invention have excellent water absorption properties and areunlikely to produce dust, and therefore applicable to waterabsorbing/retaining agents for various purposes.

Some examples of applications are water absorbing/retaining agents forabsorbent articles, such as disposable diapers, sanitary napkins,incontinent pads, and medical pads; agriculture/horticulture waterretaining agents, such as bog moss replacements, soil conditioners,water retaining agents, and agricultural effect keeping agents; waterretaining agents for construction purposes, such as dew inhibitors forinterior wall materials and cement additives; release controllingagents; cold insulators; disposable pocket stoves; sludge coagulatingagents; food freshness retaining agents; ion exchange column materials;sludge/oil dehydrates; desiccants; and humidity conditioners.

The water absorbing agent of the present invention is especiallysuitable for use in disposable diapers, sanitary napkins, and likesanitary/hygienic materials for absorbing feces, urine, or blood.

The invention claimed is:
 1. A water absorbing agent containing waterabsorbing resin particles, the agent satisfying conditions (A) to (H)below: (A) the water absorbing resin particles are, on a surfacethereof, either crosslinked or coated with a surface crosslinking agentwhich has at least one hydroxyl group; (B) the water absorbing resinparticles contain, on a surface thereof, a trivalent water-soluble metalsalt and water-insoluble inorganic particles that include silicondioxide; (C) the water absorbing agent has a mass median particle sizeof 200 μm to 500 μm inclusive; (D) the water absorbing agent contains0.001 to 0.4 mass % inclusive, of the water-insoluble inorganicparticles; (E) the water absorbing resin particles have a size such thatno more than 5 mass percent of such particles can pass through a sievehaving a mesh opening size of 150 μm; (F) the water absorbing resinparticles contain acid groups that are neutralized in a ratio from 50mol% to 90 mol% inclusive; and (G) the water absorbing resin particlescontain 0.001 to 5.0 mass % inclusive, of the trivalent water-solublemetal salt; (H) the water absorbing agent contains dust in an amountthat is 355 ppm or less and that contains SiO₂ in an amount which is 50mass % or less.
 2. The water absorbing agent of claim 1, wherein thewater absorbing agent has a centrifuge retention capacity of 30 g/ginclusive to 50 g/g exclusive, and a saline flow conductivity (SFC) of10 (10⁻⁷·cm³·s·g⁻¹) or more.
 3. A water absorbent core, comprising thewater absorbing agent of claim
 1. 4. A method of manufacturing a waterabsorbing agent containing water absorbing resin particles having a massmedian particle size of 200 μm to 500 μm inclusive, comprising: (1)polymerizing a water soluble unsaturated monomer to obtain waterabsorbing resin particles; (2) either crosslinking or coating the waterabsorbing resin particles on a surface thereof with a surfacecrosslinking agent which has at least one hydroxyl group; (3)mechanically damaging the water absorbing resin particles, so as toirregularly pulverize such particles; and (4) mixing a trivalentwater-soluble metal salt with the water absorbing resin particles; (5)further mechanically damaging the water absorbing resin particles mixedwith the trivalent water-soluble metal salt; and (6) mixing the waterabsorbing resin particles with water-insoluble inorganic particles thatinclude silicon dioxide, wherein the water absorbing agent contains dustin an amount that is 355 ppm or less and that such dust-contains SiO₂ inan amount which is 50 mass % or less.
 5. The method of claim 4, whereinthe water absorbing agent has a centrifuge retention capacity of 30 g/ginclusive to 50 g/g exclusive, and a saline flow conductivity (SFC) of10 (10⁻⁷·cm³·s·g⁻¹) or more.
 6. The method of claim 4, wherein themixing the trivalent water-soluble metal salt comprises mixing thetrivalent water-soluble metal salt as an aqueous solution with the waterabsorbing resin particles.